[Dasha]

Originally Posted by Acheron

What about killing in the defence of other people? Imagine that you know that someone is going to kill a group of your friends. You track down this person and kill him before he can kill any of them. You must have intended to kill this man, which would fit your definition of murder (technically speaking, a murder is an unlawful killing with intent). Was killing him justified?

No, that does not justify the murder at all. A person could just as easily, call up the police to track the potential murderer down, or if they don't want to,
Call the friends and tell them to get somewhere safe, and track down the potential murder and tie him up or something, THEN call the police. There is no need to murder them and there's not much way to possibly tell if the person was just making a threat, or actually going to do it.