The real issue at hand is whether or not we’re talking about moral relativism or absolutism.
If we are endorsing relativism, then all actions have a relative frame of reference by which they are justified (i.e. Bob’s killing Jane).
My stance is that in terms of absolutism, there is no such thing as justified mass violence, and that while it is certainly possible for mass violence to be a lesser evil absolutely, and thus easily argued as a moral good relative to the alternative, that ultimately it remains an evil under all circumstances objectively, and at best can be a lesser evil regarded absolutely.