In the OP, I meant deliberate extermination.
So you mean an attempt by the US government to totally, systematically exterminate all the natives?
I can only see anything close to a death camp scenario happening once the natives are in reservations, and by that time, what's the point in killing all of them? The only incentive that can exist for such a policy change would be an ideological one, since they're already out of the way of settlement and pose no militaristic threat, and there's no way to turn killing them all into money (that I can think of). Americans at this time did not want to kill all the natives - they saw them as noble savages, not dangerous monsters that need to be put down. Something has to wildly shift public opinion.
For this scenario, you need the American public to be scared of the natives when they already have them in reservations. You need a terrorist.