What is the ruling of Islamic law concerning the killer of a civilian citizen?

Unlawfully killing a soul is one of the gravest sins. Allah Almighty has forbidden it, and the Prophet, peace be upon him, warned against it. Whoever intentionally kills a believer, their recompense is Hell, and the wrath of Allah is upon them. Their obligation is retribution (qisas) unless the victim's guardians pardon or accept blood money (diyah). However, if the killing was by mistake or semi-intentional, the obligation is blood money and expiation (freeing a believing slave, or fasting for two consecutive months for those who cannot find one).