The principal was both right and wrong. He was right that this kind of drawing merits a strong reaction on the part of the school. Make no mistake: With today's culture of violence, propagated by TV shows, violent video games, and the like, kids are picking up the message that violence, shooting guns, and solving problems by beating the shit out of someone is perfectly normal.
Let's make clear to our kids: Despite what they see on TV and in the movies, it's wrong, immoral and terrible when people blow other people's brains out. Even in the context of the innocent make-believe world of children, it's not normal for children to draw this kind of drawing. It's a sign that something is wrong. This kid is going in the wrong direction, and the school - if it is to fulfill its function - must intervene.
Having said that, I don't think suspension was the right way to go about it. It's an extreme reaction with no real educational value. I would prefer the kid being called in to the principal for a chat, the parents of the kid notified and called in too for a meeting, perhaps discussing with parents the need to eliminate TV and video games from the boy's life, or at least allowing him to watch only certain, approved content. If we don't nip those violent tendencies in the bud, we're going to get more Virginia Tech, Columbine, Nebraska Mall, Colorado Church and who knows what else in the future.