The difference is, she's a Deatheater with a conscience.

This regime has made all of us do dreadful things. But sometimes you have to choose between two dreadful things, and you are forced to decide on the one that is just a little less dreadful. That's what Minerva did for us, taking the Mark. It was the right thing to do, and I'll always be grateful to her that she was brave enough to do it. I mean that, Minerva.

And that's why I think taking this kid to Moddey Dhoo is the right thing to do, too.