Murder is
never justified under the input circumstances existing in the closed-off setting of Umineko. There is not one single valid reason for any person on the island to take the life of any other person, unless that death was because the person killed was going to kill others themselves (i.e. self-defense, but then we just shift the question of why murder was planned to the person who was killed).
I mean, murder may be
understandable, but it's rarely if ever
justified. If the true culprit has an intent to kill
everyone, it's even less likely that there is any valid justification. No matter who you have a grudge on, even in broad strokes, there will always be somebody
you have no reason to kill:
- If you hate the family, then leave the servants out of it. They just work here. Even if you think they're pawns of the family, Gohda isn't, so at least one of them has nothing to do with that grudge.
- If you hate the siblings, there's no reason to harm their children. In fact, the Ushiromiya Group might well be better off under their guidance, as they at least initially appear to be better people than their parents.
- If you for some reason want to kill the servants, pick them off when they're off duty. Why involve the family?
- If you hate Kinzo... well, he's dead. Sorry.
You can start spinning a line about how you had particular beefs but other characters "got in the way," but at that point you're just admitting that you're a sociopath who trapped him/herself in a situation where those killings of necessity were required to get away with your crime.
In short, the
very best a culprit can hope for is a justified - but still wrong - reason to kill
every single person, as failure to justify even one killing means you intended that somebody die who had nothing to do with it. The old "Love Killer" theory is about the only thing that ever did this, and it might still exist as the "Quantum Suicide Beatrice" idea that cropped up a few pages back. Otherwise I really can't think of a good justification for any one character that isn't pretty much flat-out sociopathy and evil.