Aphrael (@plantmoretrees)

What I mean by "dark arts" isn't so much danger, although they are dangerous. It's more of a broad class of techniques that are used against other people to their detriment. And maybe Cain was called to defend against the dark arts, but it seems he embraced them, given the choice. One of the most dangerous parts of trying to teach others to defend against the darkness is you have to first show it to them, and by showing, tempt. If you're lucky, they learn their lesson like Mickey as a sorcerer's apprentice, but sometimes you get a Voldemort. But even if you can get a Voldemort, I don't think that should stop you from teaching DADA. A snake will always find its way into the garden, and it's better to be prepared to handle it than caught unawares.