I've had many arguments with different people in my family concerning this subject. They would have me believe that the natives were brutally killing each other long before any colonists arrived. It's not that i believe there wasn't any infighting, i just dont believe it was as prominent as some say. Any help is much appreciated!
Was there warfare in the Americas prior to the arrival of Europeans? Yes, of course. The issue of brutality, however, is completely subjective and, depending on the exact nature of the arguments you've been having, probably irrelevant. Was the run-away Mourning Wars between the Five Nations leading up to the formation of the Haudenosaunee (Iroquois Confederacy) any more brutal than, say, the War of the Roses?
Anyhow, without knowing more about what sort of information you're looking for, I'll just provide some links to previous related topics I've answered before. Be sure to check out the entire conversation for other posts on the topics.