Well, I think we do have to try and explain it, rather than leaving it at 'the Americans were evil, so they killed Natives'. For example, the two societies had different ideas of what 'respecting each other's territory' meant: nomadic tribes held as 'theirs' far more land than that which was actually occupied by their people, because of the need to move around to hunt buffalo, and so objected rather strongly when American settlers going to California or Oregon drove their wagons through certain patches of empty countryside - which looked, to your average down-on-his-luck urban worker going across the USA with little money and very little clue, exactly like any other stretch of empty countryside. Their way of objecting, at times, was to kill those settlers. Of course, this doesn't mean that ethnic cleansing is any less wrong, but it does mean that we can understand the past, rather than refusing to empathise with the motivations of people involved and putting it all down to 'evil'. If nothing else, that latter approach makes it all to easy for atrocities to happen again.