Being a victim does not cleanse your sins. I'm sure that the Natives Americans were not perfect human beings. No one is. Being killed does not make the Natives good but the Europeans bad. I hope for you that you agree with that last sentence.
The natives being killed does not make Europeans bad. Doing the killing does. (if we have to speak in such general terms - talking logic here, not actual history).
What if Bob had murdered 1000 children, and Frank wasn't a murderer until he murdered Bob?
Does being a victim cleanse your sins?