Given that Cain killed his brother, what is his fate (Heaven or Hell)?

There is no explicit evidence that specifies Cain's fate, but he disobeyed God by committing a great sin. Scholars of exegesis differed on whether he was a disbeliever (kafir) or merely a sinner ('asi), with many preferring the view that he was a sinner and not a disbeliever, and thus will not abide eternally in Hellfire.

The verses and hadiths warn the killer of a believer and assign to Cain a share of the sin for every soul unjustly killed. The repentance of a killer is accepted if it is sincere, and for one who dies upon faith without having repented, his affair is subject to the will of God.