It's like the old man who sells him Christine says, "no one gets between the guy and Christine." Paraphrasing.
Christine is like a jealous girlfriend. She requires his full attention, all his love, all his money. He can't have friends, a girlfriend...ecetera.
For his absolute devotion, he gets protection from bullies and self-confidence.
Like crossing a deal with the devil with a psychotic love story.
Christine tries to kill the girl because she's competition.
She kills the bully because in her own twisted way, she loves her man.