Is Murder in Islam a Major Sin?
Question

I took part in a murder, but I was not caught for this crime. I want to expiate for my sin. Will Allah accept my repentance without me having to hand myself in to the police?
Praise be to Allah.Murder in Islam: One of the greatest major sins
Murder (killing a person deliberately), if the victim is a believer, is one of the greatest of major sins. Allah says (interpretation of the meaning):
“And whoever kills a believer intentionally, his recompense is Hell to abide therein; and the Wrath and the Curse of Allah are upon him, and a great punishment is prepared for him” [al-Nisa 493]
The Prophet (peace and blessings of Allah be upon him) said:
“A man will continue to be sound in his religion so long as he does not shed blood which it is forbidden to shed.”
Rights connected to killing a believer deliberately
If you killed a believer deliberately, then there are three rights which are connected to that: the rights of Allah, the rights of the victim and the rights of the victim’s next of kin.

With regard to the rights of Allah: if you repent sincerely to your Lord, then Allah will accept your repentance . Allah says (interpretation of the meaning): “Say: “O ‘Ibadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily, Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful” [al-Zumar 39:53]
With regard to the rights of the victim, he is not alive so you cannot put things right with him. The matter has to wait until the Day of Resurrection, i.e., the settling of scores with you on behalf of the victim will take place on the Day of Resurrection. But I hope that if your repentance is correct and is accepted by Allah, then Allah will compensate the victim with what He wills of His bounty until he is satisfied, and you will be reprieved. 
With regard to the rights of the victim’s next of kin, which is the third right, you cannot be absolved of this until you hand yourself over to them. Therefore you have to hand yourself over to the victim’s next of kin, and tell them that you are the one who killed him, then they have the choice. If they want to they can exact vengeance upon you, if the conditions of qasas are met; or if they want to they may take the diyah (blood money) from you; or if they want to they can forgive you. 

And Allah knows best.