Xander didn't actually eat the principal. He almost gets it as bad as Spike--worse in some ways. However, he was responsible for Sweet coming to town, and not opening his mouth when people started burning themselves to death. The entire Scooby gang, minus Spike, would have been responsible for Buffy's third death by burning to death, and Spike was the only one to attempt to stop her. Willow however, did kill people, and tried to destroy the world. She got off pretty easy. And let's face it, her witchy powers make her more than human.

I agree about the comics though--and yet, I still buy them. It's a vicious cycle. They are crack magic in paper form.