In the Theravada tradition, there's no justification for killing. But justification is a personal thing. Buddhism tells you about the consequences of an action. Not whether it is justified or not. Killing intentionally is always bad Karma, whether you, the society, a religious community, a religious book, judges of courts, a god, inner voice, patriotism, honor etc. justify it or not.
But one highlighting factor is that if you kill virtuous beings, you get a lot of bad karma, whereas killing an immoral being will result in comparatively less bad karma. A person who is trying to kill you is already an immoral being. And you may think of a justifiable reason to kill him. But killing him is still bad Karma which can cause much suffering to you in the future as well as corrupting your present mindstatemind state.
Karma is a law of nature. It is not a god who punishes you depending on how justified your reasons are for doing something. ex: Think of a newborn baby touching fire. Having no knowledge of fire, is it fair for it to get burnt? Is it justified? Probably not! But why does it still get burnt? Because fire doesn't judge. It's simply a process of causes and effect. So is Kamma.