Narrated by Abdullah (Radhiallaho anho): Allah’s Messenger (sallallaahu ‘alaihi wasallam) said, “The blood of a Muslim who confesses that none has the right to be worshiped but Allah and that I am His Apostle, cannot be shed except in three cases: In Qisas for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims. Sahih Al-Bukhari Vol. 9 : No. 17

Killing an innocent non-Muslim under Muslim protection
Narrated by Abdullah bin Amr (Radhiallaho anho): Allah’s Messenger (sallallaahu ‘alaihi wasallam)said, “Whoever killed a Mu’ahid (a person who is granted the pledge of protection by the Muslims) shall not smell the fragrance of Paradise though its fragrance can be smelt at a distance of forty years (of traveling).” Sahih Al-Bukhari Vol. 9 : No. 49