"I won't tell them," he said, because it was important that Snape believed him.
"I'm also not going to cast Dark magic in front of people. I'm not going to Akzaban. Spending the rest of my life in that place isn't exactly a better option than dying," he pointed out. "Still, I'm sure there's plenty that you can teach me and it's not classified as dark." Maybe it was time to do his own research on the subject, see what he could and couldn't cast.