It depends on what you believe to be wrong.
If you thinking killing of any kind to be wrong, then you could justify killing a mass murderer as being Wrong + Wrong = Right. But from my point of view, killing isn't necessarily wrong, so it's all about personal belief.
Or, think about this. They always say, is it wrong for a poor man to steal a loaf of bread to feed his family? Let's assume this time, that there are two poor men. Poor man A and Poor man B. There's also Rich Man, we'll call him Man C. Man A, steals Man B's single loaf of bread. Without a way to feed his family, Man B then steals a loaf of bread from Man Cs Bakery. Two men doing wrongs, but now both family's are fed and Man C is still fine.
Really weird analogy I know, but I guess you could justify it as two wrongs = right.