If you're right about their alignments and abilities, you don't need to kill them. They'll do it themselves.
Attacking the Queen during a public event, for example, will likely do the trick.
Aside from her personal guard and champion, there's everyone who dreams of being knighted for saving her life. Are you familiar with the swarm rules?
You want to be the instrument of their destruction? Help them plan something suicidal, then stand back and watch it all go wrong. From what you've described, you already started down that path when you "helped" them design their characters.
What you really need to do, though, is figure out what you'll be doing afterwords, when your friends are all mad at you and nobody wants to play in any game you're in. That's the real challenge.