Is it permissible to fight someone who intends to kill me with a weapon?

It is legislated for a Muslim to repel an aggressor who assails his person, property, or honor, using the lightest means available, then progressing to the next lightest. If the aggressor cannot be repelled except by killing him, then it is permissible to kill him, and there is no sin upon the one who does so. The ruling on repelling an aggressor varies depending on the type of aggressor. Repelling a disbeliever or anyone whose blood is deemed forfeit is obligatory. As for a Muslim whose blood is not deemed forfeit, repelling him is not obligatory; rather, one may surrender to him or repel him. Surrendering to him may even be sunnah, following the example of Uthman, may Allah be pleased with him.