What is the ruling on intentionally killing a Muslim or a non-Muslim in Islamic Sharia?

Killing a Muslim intentionally is one of the gravest sins. It incurs rights related to Allah, the killer, and the slain. The right of Allah is waived through repentance. The right of the slain's guardians is waived through retribution (Qisas), blood money (Diyya), or forgiveness. As for the right of the slain person himself, there is a difference of opinion among scholars, but it appears that it is waived in the Hereafter. As for killing a non-Muslim, it is divided into two categories: A belligerent disbeliever may be killed, while a non-belligerent one may not be killed, such as a dhimmi (protected non-Muslim) or someone with whom a treaty has been made. The blood money for a dhimmi is half the blood money of a Muslim, and it requires expiation by freeing a believing slave or fasting for two months. A Muslim is not killed for killing a disbeliever, according to the preponderant opinion.