"We have forbidden magic too, but it's forbidden because it's for really evil stuff, like shutting people up in ice coffins until they die."

Absolutely nothing Popp says about the Dark Army and their various deeds is going to make them seem any less like what Quentin is taking them for.

"You can already set things on fire by accident, so what's wrong with learning how to do it on purpose? It's not like you're a bad guy."

He doesn't have anything in particular to base this on. Just, you know. He seems fine! Popp trusts his instincts.