There are lots of great reasons to be angry about your current conditions _and_ those things that have gone on before. It just doesn't accomplish much. It creates a culture of anger, vengeance, victimhood, and nihilism.
I think the danger here is to confuse the issue and start talking about who really has a right to be upset or what kind of justice is demanded by current conditions. That's a completely different conversation -- one that must happen in all of these cases, but one which happens in a lot more productive manner when people forgive. Take a look at what happened in South Africa, for instance. Not a perfect resolution, but they are not spending the next 100 years in blood feuds either.