Responses
It's because, despite putting up a front, he's just as unhinged as she is. I actually don't think he planned on breaking her so completely. He liked her as she was, I think what made him snap completely was seeing her with the other dude. Logically he knew she was loyal to him but his fucked off brain took, "I trust you but not the men around you" to whole new level and he broke her rather than risk that ever happening again. He already knew what she would do if he pushed her the right way, so that's what he did and he got what he wanted in the end.
I feel like there was no need to ruin her for no one else to want her. She was loyal to him since the beginning and was loyal to him til the end. I understand that he didn't want anyone else to approach her (and no one ever will) but goddamn she tried to prove her loyalty to him in every way and he resorted to this