Sahih al-Bukhari Book 87, Hadith 2
Allah's Messenger (ﷺ) said, "A faithful believer remains at liberty regarding his religion unless he kills
somebody unlawfully."
A man said, "O Allah's Messenger (ﷺ)! Which sin is the greatest in Allah's Sight?" The Prophet (ﷺ) said, "To set up a rival unto Allah though He Alone created you . " The man said, "What is next?" The Prophet (ﷺ) said, "To kill your son lest he should share your food with you." The man said, "What is next?" The Prophet said, "To commit illegal sexual intercourse with the wife of your neighbor." So Allah revealed in confirmation of this narration:-- 'And those who invoke not with Allah, any other god. Nor kill, such life as Allah has forbidden except for just cause nor commit illegal sexual intercourse. And whoever does this shall receive the punishment.' (25.68)
One of the evil deeds with bad consequence from which there is no escape for the one who is involved in it is to kill someone unlawfully.