the punishment for one who purposely slays a believer will be to live in hell fire forever. god is angry with him and has condemned him. he has prepared for him a great torment.
ومن يقتل مؤمنا متعمدا فجزاؤه جهنم خالدا فيها وغضب الله عليه ولعنه وأعد له عذابا عظيما