What is the ruling on attempted murder, and what is the ruling on someone who kills a person who tried to kill him?

An attempt to kill, even if unsuccessful, does not make the aggressor's blood permissible. Whoever kills him is sinful and subject to retribution, unless it was in the context of repelling an aggressor, provided that the repulsion is done with the least harmful means first, then the next least harmful. If the aggressor cannot be repelled except by killing him, then there is no sin on the killer, and the aggressor's blood is in vain.

Attempted murder is a sin for which there is no prescribed punishment (ḥadd) nor expiation (kaffarah). Its punishment is discretionary (taʿzīr), to be determined by the Imam. If the attempt results in an injury that necessitates retribution (qiṣāṣ), such as the cutting off of limbs, then the victim has the right to retribution from the aggressor, and the retribution is exacted by referring to the Imam. If the injury does not necessitate retribution, then the victim has the right to blood money (diyah) or a discretionary compensation (ḥukūmah). As for an attempt that leaves no trace, its punishment is discretionary (taʿzīr).