What happened to the Native Americans can't be undone. Just like we can't go back and undo slavery in the U.S.
However, we can strive to make sure that nothing like that ever happens again.
Let's not pretend that the natives were living in peace until white man came along. They were killing each other and taking each other's land long before those ships arrived.
I also think it's unfair to use slavery as an example of white cruelty. It was black men who sold them to us in the first place, and it was white men who eventualy set them free. Africa today still has slavery without white men having any hand in it.