Muhammad said if someone leaves Islam, to kill him
Ali burnt some people and this news reached Ibn 'Abbas, who said, "Had I been in his place I would not have burnt them, as the Prophet said, 'Don't punish (anybody) with Allah's Punishment.' No doubt, I would have killed them, for the Prophet said, 'If somebody (a Muslim) discards his religion, kill him.'" Volume 4, Book 52, Number 260, Also, see Volume 9, Book 84, Number 64.