In Goloka Vrindavana in the Spiritual Sky there are no demons, only when Krishna comes to the material creation as His Vishnu-tattva expansion, who plays the part of Krishna, does Krishna kill demons

Krishna as His original Form NEVER leaves Goloka-Vrindavana.

The Krishna that does come to the material universe, is actually a Vishnu-tattva expansion of the original Krishna in Goloka-Vrindavana who plays the part of child Krishna in the facsimile of the real Vrindavana in the material creation and even kills demons.  

However, in Goloka Vrindavana with the original Krishna there are no demons there.

Only in the material creation do demons exist who are killed not by Krishna directly, but by His Vishnu-tattva expansion who is playing the part of the original Krishna in the material creation.

Krishna is the original source of all Narayana or Vishnu forms who are all His expansions residing in the quadrillions of Vaikuntha Planets that surround His central abode of Goloka Vrindavana. 

Krishna further expands, via His first Vishnu-tattva expansion, Balarama as Maha Vishnu (Karanodakshayi Vishnu) who creates and manages the unlimited material universes called Brahmandas.

Krishna is the Supreme Personality of Godhead and cause of all causes, who resides in His personal abode known as Goloka-Vrindavan and NEVER leaves.

However, because Krishna is also the original source of all Narayana or Vishnu forms, who are all His expansions that reside in the quadrillions of Vaikuntha Planets that surround His central abode of Goloka Vrindavana.

Krishna also expands from His Vishnu-tattva form as copy of the original Krishna who then enters the material creation.

This Krishna is a Vishnu-tattva expansion from the original Krishna who is always in Goloka-Vrindavana and NEVER leaves.

Krishna further expands into the material creation, via His first Vishnu-tattva expansion, Balarama as Maha Vishnu (Karanodakshayi Vishnu) who creates and manages the unlimited material universes or material Worlds called Brahmandas.

These unlimited number of Brahmandas, coming from the Body of Maha Vishnu's pores and breathing, are massive individual material universes made up of material qualities (both subtle material energy and gross material elements), and all have a secondary material inner universe deep within each Brahmanda.

Each massive Brahmanda universe covers, encases and surrounds the inner material universe inside them with 7 layers of material qualities.

Maha Vishnu expands again as Garbhodakashayi Vishnu who creates this secondary inner material universe deep inside of the Brahmanda.

Then from Garbhodakashayi Vishnu, Brahma manifests who creates the different Planetary Systems within each inner material universe.

These Planetary Systems are spread through out the inner universe and appears as a flat looking disc made up of all the planets from all the many Planetary Systems.

All the Planetary Systems spread to nearly the edge of the inner single material universe where it meet the surrounding Brahmanda, as explained in Srimad Bhagavatam.

Because of the massive solid 7 material elements of the Brahmanda that surround the material universe within the Brahmanda, no one inside this inner universe can see outside of that confined universe as Srimad Bhagavatam also explains.

This means from within our small material universe inside our Brahmanda, we cannot see the billions of other Brahmanda's around it.

So to be clear, deep inside our Brahmanda, in the secondary universe within it, Brahma creates 14 different Planetary Systems.

Each inner material universe within their Brahmanda, has a different number of Planetary Systems according to their size.

Karanodakshayi Vishnu (Maha Vishnu) is actually sleeping and is always DREAMING the activities within each of the "secondary material universes" inside their Brahmanda.

Those dreams Maha Vishnu has, are all the activities happening within each of the Garbhodakashayi Vishnu's secondary universes.

In painting below Karanodakshayi Vishnu (Maha Vishnu) is seen laying down sleeping and Garbhodakashayi Vishnu is seen within each Brahmanda.

When Maha Vishnu dreams, He first creates the entire material universe known as the Maha-tattva (matter) which takes up 25% of the Spiritual Sky or Brahmajyoti.

The other 75% is the active Spiritual World (anti-matter) that is ever fresh, always youthful and never ending, known as the perpetual Vaikuntha Planets of Vishnu and Krishna's central abode of Goloka-Vrindavan.

Within Maha-Vishnu's 25% of the Spiritual Sky called the temporary material creation, we find is always in a state of decay, decline, impermanence.

It is where jiva-soul who have chosen to enter the material creation, are forced into repeated birth and death, forced to possess (or take) temporary material bodily vessels or containers.

The visiting souls in the material creation are forced to take repeated births again and again because the material bodies they are in simply wear out, get old, break down and eventually cease to operate.

So lets make it clear -

1 - Within this temporary material creation, we find unlimited individual massive single Brahmanda universes coming from the Body of Maha Vishnu.

2 - And deep inside each of those massive Brahmanda universes, there is a secondary smaller universe where Maha Vishnu expands Himself as Garbhodakashayi Vishnu.

3 - And from Garbhodakashayi Vishnu, Lord Brahma appears from His naval at the top of a lotus flower.

4 - Then Lord Brahma builds the many Planetary Systems inside each of the inner secondary universes deep inside their individual Brahmanda.

There is only one Maha Vishnu in the material creation yet an unlimited number of Garbhodakashayi Vishnus and Lord Brahmas.

Also it is important to understand, the amount of Planetary Systems deep inside each Brahmanda, vary in size, some larger and some smaller than others.

Our material universe within our Brahmanda has 14 Planetary Systems and is considered a small universe explained in Bhagavatam as only 4 billion miles in diameter.

Some Vedic Cosmologists think this figure of 4 billion miles in diameter describing our material universe inside our Brahmanda, is way too small of a figure and must have more to it.

Some Vedic scholars further suggest the figure of "4 billion miles" or "500,000 yojanas", as decribed in the 5th Canto of Srimad Bhagavatam, is really a code that needs decoding.

This is because they claim that just our Solar System is said to be 20 billion miles in diameter,  which is far bigger than the mere 4 billion miles diameter our ENTIRE material universe inside our Brahmanda is supposed to be. 

Another condition is measurements can be different on all the different Planetary Systems, just like the movement time is also different within all the planetary systems.

For example, 100 years on the Brahmaloka planet on the highest planetary system is 311 trillion and 40 billion human years long.

So there maybe far more to this figure that claims our material universe is just 4 billion miles in diameter.

Other universes inside their Brahmanda, have many more Planetary Systems than just 14, and are therefore much bigger than 4 billion miles in diameter.

The fact is, the size of each Brahmanda and their inner universe, depends on the how many heads Lord Brahma has.

Some Lord Brahmas, like ours, has only 4 heads, while other Brahma's within their universe inside their Brahmanda, may have 10, 20, 50, 500, 1000, 100,000 or even a million or more heads.

In this way, all the quadrillions of Brahmanda universes coming from the Body of the sleeping Maha Vishnu (as seen in painting below), ALL have a secondary inner universe deep inside it also seen in painting.

Each massive Brahmanda universe has 7 material coverings or layers that surround and enclose each of the inner secondary material universe.

This means the massive outer Brahmanda surrounding universe we are deep within, is 44 quadrillion 444 trillion 444 billion miles in diameter, or twice that size if the measurement is from the outer edge of the inside secondary universe.

If that is true then the full size of our Brahmanda is double the figure above or 88 quadrillion 888 trillion 888 billion miles in diameter.

The sleeping Maha Vishnu (Karanadakashayi Vishnu) expands Himself into the inner secondary universe deep inside each Brahmanda, as Garbhodakashayi Vishnu.

This secondary material universe is in the centre of each massive Brahmanda universe.

In the Bhagavatam, Bhu Mandala is the “earth mandala” (not to be confused with our small earth globe), is a disk 500 million yojanas in diameter (4 billion miles in diameter)

The word ''Earth'' has many meanings.

For example, our small Earth global round sphere is only 24,901 miles in circumference and is 7,917.5 miles in diameter, it is NOT Bhu-Mandala.

Bhu-Mandala is 500 million yojanas in diameter (4 billion miles in diameter or 12.6 billion miles in circumference).

A yojana is a unit of distance about 8 miles long, and so the diameter of Bhu Mandala is about 4 billion miles which is the size of our single universe.

Our Earth is NOT flat! It is a round global sphere.

Srila Prabhupada - "On the ground our earth looks flat but when you go up you will see our Earth planet is a ball" (Lecture New York 1966.)

The Sanskrit word "Dvīpa" has two meanings.

1 - "Islands" or "continents" on a Planet.

2 - "Globes or spheres" in outer space.

Bhu-Mandala is marked by circular features designated as islands (Dvipas that can also mean planets) and oceans.

Srila Prabhupada -"The "planets" are called "dvīpas". Outer space is like an ocean of air. Just as there are islands in the watery ocean, these planets in the ocean of space are called dvīpas, or islands in outer space" (Caitanya Caritamrita Madhya 20.218, Purport)

Therefore Bhu-Mandala is far too big to be our small earth globe.

But this does describe our small single universe with its 14 planetary Systems deep within our surrounding Brahmanda.

Therefore the 14 planetary Systems deep within our surrounding Brahmanda, would therefore fit in the circumference of Bhu-Mandala (4 billion miles (500,000,000 Yojanas) diameter)

The jiva soul can stay for an almost eternity in the temporary decaying material universes cycles of annihilation and creation if they choose.

When Lord Brahma dies after 100 of his years (311 trillion and 40 billion human years), there is an equal period of his life span of 311 trillion, 40 billion human years where nothing living exists, everything in the material universe remains dormant for that enormous amount of time.

Then the next material creation of Brahmanda's manifest from Maha Vishnu (Karanadakashayi Vishnu)

All jiva souls have merged as "individual units" into the Body of Karanadakashayi Vishnu (Maha Vishnu) and remain that way until the next creation of Brahmandas when they again manifest from Maha Vishnu's Spiritual Body as seen in painting below.

In other words, in the next new creation of Brahmandas coming from the Body of Maha Vishnu, all the dormant jiva souls awaken and continue on again within a secondary universe that is deep inside each Brahmanda outer universe, from where they left off in the previous inner material universe.

As explained, a secondary smaller universe is deep within each of the massive surrounding Brahmandas.

All lasts for 311 trillion and 40 billion human years, or the life span of Lord Brahma.

It is important to understand at the annihilation of each Brahmanda universe, and its inner secondary universe, Garbhodakashayi Vishnu temporarily merges back as one with Maha Vishnu until the next Brahmanda universes manifests from the breathing and pores of Maha Vishnu.

Then the process repeats itself, the sleeping Maha Vishnu (Karanadakashayi Vishnu) expands Himself into the inner secondary universe that is deep inside each Brahmanda as Garbhodakashayi Vishnu that is in the centre of each Brahmanda universe.

The jiva souls are suspended in ''inactivity'' (dormant) within the Body of Karanadakashayi Vishnu (Maha Vishnu) for 311 trillion 40 billion human years.

The jiva souls re-emerge and continue on (their karma) from where they left off in the previous creation.