Ending Interpretation....... SPOILERS

I don't think David was that devious, Alan tried to put him with the gal to get David out of the way. He never nor anyone else set up Alan, but when he had his chance to get rid of him he took it buy snitching on him. Alan treated him like dirt anyway, Hey that's just teenagers. In the end it was Alan who plotted revenge on the gal. We never really are given his real reason for killing Alan weather, it was to get rid of him once and for all or truly out of fear of what he might do, remember Alan felt he was set up by the girl....way to many different versions on the ending then what It really was. Who cares if it was that David wasn't a wimp or who was really the bad/weird one. That, in my opinion has nothing of what the writers were trying to say. If anything, It said the one who had eyes for her and may have deserved her got her in the end. No more no less......The one point that for me that did not add up is that David became a more cocky person as the film progressed, he had enough bad ass in him to have taken Alan on earlier but never did. If you can shoot and kill someone you can tell them to get lost and make him believe it.