What is the ruling of the Sharia regarding a mujahid who kills another mujahid—after the latter presumed that he would be killed by the former—given that the killer knew the slain was a mujahid, and in this case, is fasting obligatory upon him?

Killing unjustly is one of the gravest sins. However, if the killer cannot avert the harm from the victim except by killing him, after exhausting all possible means to repel the harm from himself, then there is no sin upon him, and he is not required to pay blood money (diyah) or expiation (kaffarah). This falls under the category of repelling an aggressor (daf' al-sa'il).

Jurists have differed on the ruling regarding repelling an aggressor against oneself. The Hanafis, and the soundest opinion among the Malikis, hold that it is obligatory to repel an aggressor against oneself or anything less, whether the aggressor is a Muslim or a disbeliever, sane or insane.

The Shafi'is maintain that defense is obligatory if the aggressor is a disbeliever and the attacked is a Muslim. However, if the aggressor is a Muslim whose blood is not considered forfeit, then it is not obligatory to repel him, according to the more evident opinion; rather, it is permissible to surrender to him.

The Hanbalis hold that it is obligatory to repel an aggressor against oneself outside times of fitnah (strife/discord), based on the verse: {And do not throw [yourselves] with your [own] hands into destruction} (Quran 2:195). However, in times of fitnah, he is not obliged to defend himself.