Undecided... Why is Joe a murderer?

I can't decide how I feel about the end of The Round House. On one hand, it seems rather stupid to me that Joe actually shot Lark. I never thought he could go through with it. But on the other hand, maybe the shooting shows how violent the Native American culture was. There always seems like there are two ways of looking at the issues in The Round House. 

My initial reaction was "Whaaaa....?!" I didn't want Joe to be a murderer. But no matter how I tried to rationalize it, killing Lark was crazy. It was disturbing that he wanted to. Sure, he was protective of his mother, but murder is murder is murder. Maybe I just can't understand Joe's motivation completely because nothing so terrible has ever happened to anyone I've loved. It's one thing when my mom says, "If anyone ever hurts you, I'll kill 'em." That just what she's supposed to say, being my mother. Real murder is an entirely different matter.

However, as I thought about Joe's circumstance more, I began to realize some things. The legal system on the reservation was obviously less than great. It is entirely possible that Joe saw no other option, as his father couldn't kill the guy, and Joe couldn't trust anyone else with the dirty job. I'd like to think that in his situation, I'd at least be able to wait until Mayla was found. 

But maybe, Louise Erdrich had Joe kill Lark for an entirely different reason. We have already explored the violence and fear on Native American reservations (the video Daniel suggested in his comment on my last post was especially poignant). Maybe Joe shooting Lark is meant to show us that it's not only bad guys who perpetrate crimes in these places. It’s an unfortunate part of the culture, and if the Native Americans had better legal systems, maybe this wouldn’t be as big of an issue. Vengeance and vigilante justice are still crimes.

Whatever Erdrich’s reasons for killing off Lark, I’m still not sure about it being Joe who did it. (And Cappy. I don’t have any profound reasons yet for why Cappy shot the guy.) I’m still left with this on-edge, kind of uncomfortable feeling. If Joe could kill someone, couldn’t anyone?