Who is the Killer of Lord Krishna?
No one can kill Krishna! Krishna is not a normal person like us. Krishna’s body is not like our material body, his body is totally transcendental and purely spiritual. This is confirmed in all Vedic scriptures like Bhagavad-gita, Bhagavatam.etc
ANSWER IN DETAIL:
Before someone start to know the killer of Lord Krishna, one can have a look at what Bhagavad Gita says about the appearance and disappearance of the Lord from this material world:
ajo ’pi sann avyayātmā
bhūtānām īśvaro ’pi san
prakṛtiṁ svām adhiṣṭhāya
sambhavāmy ātma-māyayā
Although I am unborn and My transcendental body never deteriorates, and although I am the Lord of all living entities, I still appear in every millennium in My original transcendental form (Bhagavad Gita 4.6) .
The Lord has spoken about the peculiarity of His birth: although He may appear like an ordinary person, but He is not, because He does not change His body, as the common living entity changes/transmigrates from one body to another. He appears exactly in His eternal body, uncontaminated by this material world. Although He appears in the same transcendental body and is Lord of the universe, it still appears that He takes His birth like an ordinary living entity. And although His body does not deteriorate like a material body, it still appears that Lord Krishna grows from childhood to boyhood and from boyhood to youth. But astonishingly enough He never ages beyond youth. He looked just like a young man twenty or twenty-five years old, even if he had many grandchildren at home, and he was sufficiently aged as per material calculations.
He is the same unborn, eternal form of bliss and knowledge, changeless in His transcendental body and intelligence. Factually, His appearance and disappearance are like the sun’s rising, moving before us and then disappearing from our eyesight. When the sun is out of sight, one can think that the sun has set, and when the sun is before one’s eyes, one can think that the sun is on the horizon. Actually, the sun is always in its fixed position, but owing to one’s defective, insufficient senses, one start calculating the appearance and disappearance of the sun in the sky.
And because Lord Krishna’s appearance and disappearance are completely different from that of any ordinary, common living entity, it is evident that He is eternal, blissful knowledge by His internal potency – and He is never contaminated by material nature.
To conclude, although Krishna’s pastimes seem to appear and disappear, they are continuously existing in one brahmāṇḍa (universe) or another. Thus, all of Krishna’s līlās are present simultaneously throughout the innumerable universes. By our limited senses we cannot appreciate this; therefore Krishna’s eternal pastimes are very difficult for us to understand (Purport, CC Madhya Lila 2.20.397).
Who was the person responsible for Lord Krishna’s disappearance from this material world and how did it happen?
The answer is depicted clearly in a discussion that happened between the hunter Jara and Lord Sri Krishna in the 11th Canto of Srimad Bhagavatam,
muṣalāvaśeṣāyaḥ-khaṇḍa-
kṛteṣur lubdhako jarā
mṛgāsyākāraṁ tac-caraṇaṁ
vivyādha mṛga-śaṅkayā
Just then a hunter named Jara, who had approached the place (where Lord Krishna was seated under a peepal tree), mistook the Lord’s foot for a deer’s face. Thinking he had found his prey, Jara pierced the foot with his arrow, which he had fashioned from the remaining iron fragment of Samba’s club (SB 11.30.33)
Then, seeing that four-armed personality, the hunter became terrified of the offense he had committed, and he fell down, placing his head upon the feet of the enemy of the demons (SB 11.30.34)
Jarā said: O Lord Madhusudana, I am a most sinful person. I have committed this act out of ignorance. O purest Lord, O Uttamahsloka, please forgive this sinner (SB 11.30.35)
O Lord Viṣṇu, the learned say that for any man, constant remembrance of You will destroy the darkness of ignorance. O master, I have wronged You! (SB 11.30.36)
Therefore, O Lord of Vaikuntha, please kill this sinful hunter of animals immediately so he may not again commit such offenses against saintly persons (SB 11.30.37)
śrī-bhagavān uvāca
mā bhair jare tvam uttiṣṭha
kāma eṣa kṛto hi me
yāhi tvaṁ mad-anujñātaḥ
svargaṁ su-kṛtināṁ padam
The Supreme Personality of Godhead said: My dear Jara, do not fear. Please get up. What has been done is actually My own desire. With My permission, go now to the abode of the pious, the spiritual world (SB 11.30.39).
So instructed by the Supreme Lord Krishna, who assumes His transcendental body by His own will, the hunter circumambulated the Lord three times and bowed down to Him. Then the hunter departed in an airplane that had appeared just to carry him to the spiritual sky (SB 11.30.40)