I asked Ibn 'Abbas (about the verse relating to intentional homicide in Surat An-Nisa') He said: When the verse "Those who invoke not with Allah any other god, nor slay such life as Allah had made sacred, except for just cause" was revealed, the polytheists of Mecca said: We have killed the soul prohibited by Allah, invoked another god along with Allah for worship, and committed shameful deeds. So Allah revealed the verse "unless he repents, believes, and works righteous deeds, for Allah will change the evil of such persons into good." This is meant for them. As regards the verse "if a man kills a believer intentionally, his recompense is Hell" He said: If a man knows the command of Islam and intentionally kills a believer, his repentance wil not be accepted. I then mentioned it to Mujahid. He said: "Except the one who is ashamed (of his sin)."