This strikes a nerve. He doesn't doubt what she says - he's seen plenty evidence to support it. Between what she's said and what Anders told him (a slightly biased source, true, but he doesn't know that). People being seen for the worst in them and nothing else just makes him want to scream. He can't quite put into words why, but it does.

"So they lock mages up in these Circles because they're afraid of what they might do? And I heard about what Templars do to mages that step out of line. That Tranquility thing. It's just awful. I get not wanting to imitate a mistake, but you don't suppress group of people to avoid it. Because then you become a different kind of history that hopefully someday, someone else isn't going to want to repeat."