The worst abusers I've seen or heard of didn't merely claim to do it for their victims' sake, but seemed to actually believe so.
Notice that Þrym is cornering her there. Once she's ready to heal, Ellen will have to rebuild herself a different life, trying to go on as before wouldn't work. But if he orders it first, she'll have to struggle with her pride before doing it, and then live with the idea that he was right all along.
So cutting herself is an act of defiance and not self hatred? ...or is she saying "look, you can't hurt me"?
She likely didn't put much thoughts into it. She was unable to interact with her friends in a meaningful way, she realised part of the problem was that they didn't aknowledge that her handicap had changed her, and decided to make change visible. The message was for her friends, not her enemy, even though he obviously understood it better than the former.
Is it defiance? Yes, it is. She's trying to retake control of parts of her life. It's also despair.
The brutality is bone chilling...Prym is quite the monster.
Something is quite certainly wrong in his head.