Yeah, if he can magically heal people and even raise the dead (all shit that happened in the Bible), he can stop a couple of trigger-happy nutjobs. Even if you buy that not being able to stop a couple of kids with guns qualifies as omnipotent (which it doesn't), he at the very least could've brought everyone who died back to life. Not to mention, even you have to admit that would've been a far more compelling argument for his existence than a shitty little doodle in some emo kid's notebook.