Mallia Mamet was touched when she heard Steve Harris speak so calmly. Indeed, she should have understood that the reason Steve did what he did was that he cared about her.
But Mallia knew better than anyone that the kind of self-inflicted pain and indignity was addicting; she couldn't stop.
Even if each slash of the knife carved into her heart, Mallia felt it was consensual if George Flack was the one wielding the knife. Then, it became romantic, having nothing to do with cruelty.
Moreover, Mallia never cared about the consequences. She only cared about how she felt in the moment. The love she felt then was enough. There was no need to measure the pain that lay behind.
She had especially disliked Steve Harris, but now her attitude towards him had gradually changed. Appreciation mixed with a little guilt. If Steve had done so much for her, then stopping everything wasn't just for George Flack.