Nick always wanted his school principal gone, so he's glad when the dude gets eaten. That's a violent end for anyone, even someone you don't like, so it's a little shocking that it makes Nick happy. But violence isn't a big deal for him—and that's even before he finds himself needing to fight zombies.

With Nick's attitude toward his dad being totally negative, this sentence makes sense on that level. But we'll later learn that Nick's dad actually is some sort of monster. Any reference to a monster in this book is literal as well as metaphorical.

This is good advice for anyone, but it's even better advice when your friends might be demons from another dimension.