Krsna, as His original childhood Form, never leaves Vrindavana in the Spiritual world.

Krsna is the Supreme Personality of Godhead and cause of all causes, and the original source of all Visnu/Narayana forms of God known as Visnu-tattva expansions.

Therefore, when Krsna comes to the material creation, He comes via His 4 armed Visnu-tattva expansion, who plays the role of the original 2 armed Krsna from Vrindavana, in the facsimile Vṛindāvana in the material creation. 

Srila Prabhupada - "Kṛṣṇa as His original childhood form NEVER leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His Viṣṇu-tattva expansions."(BG As It Is Ch 10 text 37)

Srila Prabhupada - "The original Lord Kṛṣṇa never leaves Goloka Vṛndāvana. All the plenary expansions are one and the same Viṣṇu-tattva, and there is no difference in Their potency." (SB Canto 3 Ch 1 Text 34)

Srila Prabhupada - "Kṛṣṇa is the original Supreme Personality of Godhead, and Baladeva (Balarama) is Kṛṣṇa's immediate expansion. Because Kṛṣṇa NEVER leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His expansions." (BG, As It Is Ch 10 Text 37)

Kṛṣṇa’s first expansion is Balarama, and from Balarama ALL Visnu-tattva expansions originate.

When the "original" Krsna wants to enter the material creation to perform His pastimes in the Vrindavana facsimile of the spiritual world, His Visnu-tattva expansion, coming from Balarāma, performs those Kṛṣṇa pastimes or lilas.

In other words, because the original Kṛṣṇa NEVER leaves Vṛndāvana, His Visnu-tattva expansion coming from Balarāma, plays the part of the original Kṛṣṇa in the material creation that includes killing demons there.

Krsna's first expansion is Balarāma and from Balarama all other Visnu-tattva Forms manifest.

In other words, the Krsna who comes to the material creation, is really His four armed Visnu-tattva expansion (from Balarama) of the original two armed Krsna, now playing the part of the original Krsna in the material creation. 

This means Krsna, as His original two armed form, never leaves Goloka Vrindavana and has never been to the material creation as His original childhood form. 

This requires proper knowledge, devotion and personal realization to properly understand this. 

Srila Prabhupada - "Kṛṣṇa is the original Supreme Personality of Godhead, and Baladeva (Balarama) is Kṛṣṇa's immediate expansion. Both Lord Kṛṣṇa and Baladeva appeared as sons of Vasudeva, so both of Them may be called Vāsudeva. 

From another point of view, because Kṛṣṇa never leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His expansions. 

Vāsudeva is Kṛṣṇa's immediate expansion, so Vāsudeva is not different from Kṛṣṇa. 

It is to be understood that the Vāsudeva referred to in this verse of Bhagavad-gītā is Baladeva or Balarāma, because He is the original source of all incarnations and thus He is the sole source of Vāsudeva. 

The immediate expansions of the Lord are called svāṁśa (personal expansions or Vishnu-tattva), and there are also expansions called vibhinnāṁśa (separated expansions or jiva-tattva)." (BG 10.37, Purport)

Srila Prabhupada - "The original Lord Kṛṣṇa never leaves Goloka-Vṛndāvana. All the plenary expansions are one and the same Viṣṇu-tattva, and there is no difference in Their potency." (SB, Canto 3 Ch 1 Text 34 Purport)

When Krsna comes to this material world for His childhood pastimes, it is His Visnu-tattva expansion who plays the role of Krsna.

Srila Prabhupada - "Because Kṛṣṇa never leaves Vṛndāvana, all the forms of Kṛṣṇa that appear elsewhere are His expansions. Vāsudeva is Kṛṣṇa's immediate (Visnu-tattva) expansion, so Vāsudeva is not different from Kṛṣṇa. It is to be understood that the Vāsudeva referred to in this verse of Bhagavad-gītā is Baladeva, or Balarāma, because He is the original source of all incarnations and thus He is the sole source of Vāsudeva." (BG Ch 7 Text 2 Purport)

Srila Prabhupada - "The immediate expansions of the Lord are called svāṁśa (personal expansions), and there are also expansions called vibhinnāṁśa - separated expansions." (BG Ch 7 Text 2 Purport)

Srila Prabhupada - "Because Mathurā and Vṛndāvana are intimately connected with Kṛṣṇa eternally, it is said that Lord Kṛṣṇa never leaves Vṛndāvana (vṛndāvanaṁ parityajya padam ekaṁ na gacchati)." (SB, Canto 3 Ch 1 Text 34 Purport)

Srila Prabhupada - "Since that time, the city of Mathurā had been the capital of all the kings of the Yadu dynasty. The city and district of Mathurā are very intimately connected with Kṛṣṇa, for Lord Kṛṣṇa lives there eternally." (SB, Canto 3 Ch 1 Text 34 Purport)

Srila Prabhupada has explained that the Krsna in the material creation is actually a Visnu-tattva facsimile of the ORIGINAL Krsna in Vrindavana.

This means it is Visnu playing the role of Krsna who enters the material creation for pastimes including killing demons.

Please read Prabhupada's comments, this is what he says in Srimad Bhagavatam and Caitanya Caritamrita. 

The paradox is, even though the original Krsna never leaves Vrindavana in the Spiritual Sky and it is His Visnu-tattva expansion who plays the part of Krsna outside the original Vṛndāvana, the paradox is there is really no difference between Kṛṣṇa and Visnu.

Except the original Krsna never kills demons in the original Vrindavana, nor never leaves that Vrindavana in the Spiritual world. 

The facsimile Krsna kills the demons in the material  creation and is really Krsna's Visnu-tattva expansion.

Visnu is none different than Krsna and is as good as the original Krsna anyway.

In other words, there is no difference between Kṛṣṇa and His Visnu-tattva expansions, a paradox to many.

Krsna means the most attractive and is the original form of God, from Krsna everything else manifests.

The correct understanding of this quote from Brahma Samhita.

"I worship Govinda, the primeval Lord, who by His various plenary portions appears in the world in different forms and incarnations such as Lord Rama, but who personally appears in "His supreme original form as Lord Krsna." (CC Adi 5.155)

Photo -The "facsimile" Krsna's real form in the material world is Visnu as he plays the part of the original Krsna, who is the Supreme Personality of Godhead and cause of all causes and remains eternally in Goloka Vrindavana.**.