People sympathize with Carrie, she is a tragic monster who was pushed into her actions. If she wasn't relentlessly bullied might have never snapped and never killed anyone. We can't know. Yet her actions were still wrong, she was still a monster.
The biggest difference between someone like Carrie and Frollo is intent. Frollo wanted to wipe out everyone he saw as impure. He thought he was doing God's will. All while ignoring his own sins. Carrie was a victim pushed too far.