Yes, a lot of the natives fell victim to genocide but that's still different from the sort of free-for-all the American west was where settlement preceded law and order.
The rest of the British colonies settled by mostly Europeans: Canada, Australia and New Zealand are still less violent than the USA. The legacy of slavery is the cause of much of the violence in the US.
It isn't just slavery, though that is a big part. I think the general mindset is built on getting to the top by standing on the bodies of others.
Now for something a bit controversial: the difference between civilized and uncivilized is maybe not in the amount of killing done but in how organized the killing is. Thoughts?
Well, "sanctioned killing" in my opinion is still murder. Is there really a justifiable reason for a government to kill anyone in or outside of their jurisdiction?
That's another interesting question. I was making a cynical observation, really. What I find is that if rationalizations and justifications were rocket fuel, humanity would've been able to explore half the galaxy already.