I won't talk about LiT here again, so as not to worry anyone. See, I like it quite a bit.
See, this I totally disagree with. It does seem to be a popular interpretation, but I don't get it. I mean, ra-ra loyalty and all that, but they don't decide to slaughter the entire town out of loyalty to Angel. It's just what they do, because they're a bunch of bastards. I don't see it so much as Pike being noble and doing The Right Thing. It is important for him, personally, because he's always abandoning people, but it's just as much ennui. They've got nowhere to go and nothing to do. Time's up. If they hadn't killed Angel, I don't think it would have changed a thing.
I think our disagreement may be a little more subtle. When I said he's doing the right thing, I meant for him personally. I don't think it's so much about loyalty to Angel (although that's definitely why Mapache dies) and nobility and all that garbage as it is about Pike accepting that yes, he has nowhere to go (as Dutch says earlier in the film to his notion that they'll do this last job and back off, "Back off to where?") and time has passed him by, but by god, if he's going to kill a bunch of people, rather than doing it in another botched robbery plan or rather than killing his best friend, who he wronged, he's going to do it right there in Agua Verde because a) he's paid a lot of lip service to the notion of the Bunch staying together (which is, as you point out, quite important to him to actually do what he says for once) and b) he's driven by his disgust at the forces of progress, which, in that place, are represented by the Germans and the uniforms. I think that's why Peckinpah goes out of his way to show that it's a child who shoots him, because, as illustrated by the earlier scene in which Mapache stands the bombing by Pancho Villa in order to use the telegraph, the children hero-worship Mapache. Pike has to die because he's a relic, and he knows it, and for him, the exact right thing to do in that situation is to struggle in vain against the future, ostensibly to uphold the values that he's been paying lip-service to when it suits his purpose. He has to prove to himself that being a bastard to the world is alright as long as you're not one to your compadres. I mean, he's wrong, but that's how I read that fucking incredible look on his face in those couple of seconds between shooting Mapache and shooting the German.
Also, I should point out that Pike originally thinks of Mapache as just another outlaw like the Bunch, but Dutch points out that they don't hang people. Mapache is a completely different sort of bastard, and I think that Pike figures that out when Mapache won't sell Angel back to them.
Edit - On re-read, I'm sorry for the convoluted sentences. If I could make heads or tails of them, I'd rewrite this for clarity.