10 Worst Things The Joker Has Ever Done To Harley Quinn
2. He Trapped Her In An Abusive Cycle
The Joker's continued abuse of Harley Quinn is bad enough, but what makes it even worse are the tiny moments of kindness he shows her. These scraps of affection don't come from a place of genuine love, but rather they're a carrot dangled before Harley to convince her to always keep coming back for more.
The New Batman Adventures episode Mad Love and the one-shot comic it was based on (which was written in the show's continuity, making the whole thing a weird loop), is the perfect example of that.
Throughout the course of Mad Love, Joker violently refuses Harley's advances. All her attempts to seduce him are met with either verbal or physical abuse, culminating in him throwing her to the hyenas when she covers herself in pudding (for her Puddin).
This is what pushes Harley to try and kill Batman herself and well... we already know how that ended.
After her fall, Harley woke up in the hospital and completely renounced the Joker, deciding that she was going to leave him as soon as she healed. Unfortunately, she then turned her head to see a bouquet of flowers and a get well soon card from the Joker, ensuring she would go running right back into his arms.