You know, while this is an amazing response, I think no one actually talked to Belegar about miscasts and how at worst they can summon demons or open the way for Chaos. I think with how good Mat has been about her casting, he has no real practical idea how bad it is to be next to a wizard miscasting. Kragg does, he respects us despite it, but Belegar may be getting a bit stary eyed from this response.
Then again, it can kill armies. So it isn't without reason to respond like that.
I loved the final lines.
Bewildered, dismissive:"You have any other apocalypse weapons?"
Shuffling, not meeting eyes:"None ready for deployment."
Spit take, disbelief:"Wizards"