To the extent that iterated decision theory accurately models historical selection pressures which shaped our intuitions, I agree with you. However, moral positions like "violently victimizing someone from your own tribe for trivial personal gain is bad and should be heavily discouraged" have been uncontroversial since before decision theory was formalized.
Imagine a simple decision game: Should I eat the poisonous fruit: Yes (-100), No (0). Obviously, No is the superior answer, and it didn't take publication of this decision theory result for humans to realize this. Making the decision game is writing the expected payouts of the environment - not setting them.
To take your example, as long as increasing the power of the tribe provides benefits to you (and I agree that it usually will), then reducing inter-tribe squabbling is the better long-term choice. Decision theory doesn't disagree, but isn't necessar...
My intended next OB post will, in passing, distinguish between moral judgments and factual beliefs. Several times before, this has sparked a debate about the nature of morality. (E.g., Believing in Todd.) Such debates often repeat themselves, reinvent the wheel each time, start all over from previous arguments. To avoid this, I suggest consolidating the debate. Whenever someone feels tempted to start a debate about the nature of morality in the comments thread of another post, the comment should be made to this post, instead, with an appropriate link to the article commented upon. Otherwise it does tend to take over discussions like kudzu. (This isn't the first blog/list where I've seen it happen.)
I'll start the ball rolling with ten points to ponder about the nature of morality...