Narrated Amr bin Shu’aib:
From his father, from his grandfather that the Prophet (peace be upon him) said: “Whoever kills [a believer] deliberately, he is handed over to the guardians of the one killed. If they wish to, they have him killed, and if they wish to, they take the blood-money. That is thirty Hiqqah, thirty Jadha’ahs and forty pregnant camels. Whatever (amount more) they require from him, than that is for them (if they choose).” THat is because of the severity of blood-money.
Jami` at-Tirmidhi, Book 16, Hadith 2