Rita's definitely thinking about the compassion thing, but it's also true that she can't quite imagine him as an effective fighter. So it's a bit of both.

"So a spirit of compassion can hurt someone if it's to prevent greater harm... is that it?" she supposes.

Lavellan said before that one way that spirits become demons is if they're forced to act against their nature. Cole isn't being forced, and he's supposedly protected against that sort of thing anyway... but having heard that, she had to wonder if he was capable of harm, and what his limitations might be.