I actually wouldn't mind him being even angrier like that, sort of a completely dark and grizzled type. It'd certainly be justified, considering the realities of historical relations between Indians and the colonists.
But as he is, his anger just comes off as, like I said, teenage outbursts. Half the time he just sounds like one of these dumb kids you hear nowadays, reciting ideological jargon they probably heard from their parents.
And his voice is actually a big part of that, too. I don't think he was very well-cast. His voice actor has that sort of adolescent twinge to it, like it might crack any time he yells. Doesn't do him any favors.