Mary judged the man she was looking at through the mirror. He saw his girlfriend die in a dream and did nothing to prevent it. She was killing him when she felt herself smash to bits. But it wasn't over yet. Now she was free of the mirror. In pain, she crawled out and attacked both of them. The other one may not have killed people, but he killed others like her.
Then she saw herself in the mirror as the man held it up to her. She looked like a hag. It was because of all the killings she had committed. She was a hypocrite. She punished everybody else and now she had to punish herself.