Well, we don't know for sure that the monster killed Dennis.

I mean, one thing that could have happened is Dennis's magic came out so strong that it killed him.

Or Dennis could have used his magic some other way, and Ollivander killed him. I mean really, someone who's capable of doing...what he did...is capable of nearly anything, right?

Or maybe the monster killed him.

Or maybe the monster gave Dennis a heart attack.

It could make a big difference.