There's no point in holding a grudge unless it solves problems, right? What's the point of making yourself all worked up, angry, resentful, at something that you can't fix, you can't influence? What's the point of being constantly enraged by something that happened in the past, when you can't fix the past, you can only make sure it doesn't happen in the future? It doesn't matter what they did to you before, it only matters that it was fixed and it doesn't happen again.
I guess that's the thing. "You killed my father! Prepare to die!" doesn't actually solve anything. All in all, now there are two dead guys. Sounds sort of noble, though in the end nothing really was fixed. Holding a grudge like that, whatever the severity of the crime done to you, is pointless.