If someone kills a person who is planning to murder many others, but has not yet done so, is that justified, or is that also murder?
If a person is about to kill another person without justification, the Torah allows stopping that person by any means, even killing him. However there are several caveats. The danger must be clear and immanent, and the act must be unjustified. If it is any way possible to deter the murderer by other means, such as restraining or even maiming him, this should be done. The phrase "planning to murder many others" requires clarification. If the intention is that this person plans to wantonly kill, the obligation to stop him even by killing him applies. If the meaning is that he plans a course of action that you think endangers lives (such a a government official pursuing a policy that you believe endangers lives) that is certainly not sufficient grounds for applying this rule.