Basic human psychology. When you deliberately hurt someone you may hate yourself but if they get over it you usually do too. When you deliberately hurt someone and they never get over it you stop hating youself and learn to hate them. This is the basic idea behind Clifton Adams' 1953 thriller Whom Gods Destroy. The main character Roy Foley learns this lesson early by kicking a defenseless crippled dog, which he sees every day afterward and therefore keeps kicking it until it goes away. But the lesson really sinks in when his unrequited high school crush kicks him. She's from a wealthy family and he's from the wrong side of the tracks. When he confesses his love for her she laughs in his face. The next day he drops out of high school, flees town, and throws away his bright future. That's backstory. The book opens when he returns fifteen years later. He inevitably sees her again and, as the abused, hates her with a murderous intensity. And as the abuser she hates him right back. It's clear these two are going to mangle each other. Whom Gods Destroy is recommendable stuff.