Because violence is a tragedy and in an ideal world there would be no need for it. However, fewer and fewer people these days can pretend we live in an ideal world.
I do think, ideally, we should be able to resolve disputes without violence. We don't live in that world though. Mainly because people that have a lot power and resources worked to keep it that way. They actively work against progress.
I do think it's morally defensible for an oppressed group to direct violence toward their oppressors. It's also a tragedy that it comes to that but tragedy and justice are not mutually exclusive. I also think only an idiot would accept the standards in which we are expected to live, therefore to demand satisfaction with such standards would be idiotic.