After causing an accident, a poor young man finds himself bound to the increasingly chaotic whims of a bored billionaire.
I think emphasizing the accident (the incident) puts more stress (in both senses) on the nature of their relationship. Also, I'm finding narcissistic to not convey as much info. To an extent I think we expect billionaires to be narcissistic. Bored carries the billionaire into potentially darker territory.
I don't like rebellious. It feels like I've seen that guy and that sends the story into semi-predictable areas. If this were a comedy, sure, but I've got a Cheap Thrills / Cape Fear vibe that I can't shake. I don't like "poor young man" for the same reason. Malcontent still feels right. This is the downside to working from logline to script.
Below is a reverse logline from the billionaire's POV. I don't know if it's any better, but pushes towards the (shock?) ending.
After a directionless young man enters his life, a bored billionaire discovers a dark pleasure that threatens their lives and sanity.