When breaking the main character's spirit backfires horribly

Zorin Blitz from Hellsing. She showed Seras hallucinations of her parents' deaths, chopped off her eyes, and even killed her crush Pip. However, that last one was her biggest mistake since it gave Seras access to his blood, allowing her to ingest it and become a powerful vampire with enough strength to pulverize Zorin's head

Mahito from Jujutsu Kaisen. From killing Nobara and Nanami to sadistically laughing at Yuji over Junpei's death, it's no secret Mahito wants to break Yuji's spirit just to prove they're similar. However, when Yuji started kicking Mahito's ass and then proclaiming he was right about them being the same and deciding to kill Mahito, he just ran away crying like a bitch.