Indians generally had the utmost respect for nature since their lives depended on it, so they did not kill for sport as a general rule.
Some of the other posters have pointed out that there was a mix of good and bad as on any Continent. Some tribes and individuals were wise, while others were not. They were also no different from people in other parts of the world in that they were tribal and often had intertribal warfare. They were brutal and used torture freely as a general rule. This is no different from other parts of the world either. They did this because it instills fear in the others and thus promotes the safety of the tribe and reluctance to go to war, IMHO.
Supposedly, the loss of buffalo has contributed to the increase in forested land.
Don't get me wrong, I could not care less about this loss...