Whether you're talking about a fictional character (Billy) or an actual person...
How can you say, "I don't think his abusive, racist, misogynistic behavior would have improved with time--because he didn't see it as a problem. You have to want to change."
How did you know he didn't want to change? Why are you so damn certain he's incapable of change? He's literally only 18 and has the rest of his entire life to change. You think he's honestly gonna sit there and be like, "nah this is fine. I'm set. I like everything just the way it is." And never once contemplate or attempt a change.
Every person attempts change on some level. Not everyone makes big ground shaking changes. Not everyone gets to a point that is satisfactory to others. That's not the point.
When you are around real people and/or work with real people and use this language guess what happens? It makes them feel incapable. It feels undermining. It helps solidify and corner them into a box they may not want to necessarily stay or be in.
I've worked with clients with records. Clients who've said outright sexist and bigoted things in front of me and at times directed towards me. But I continued to work with them. Guess why? Because if I were to leave in that instance, I'd only be proving a point. A single point. That again there's this giant narrative of "no one believes in you", "you clearly don't want help", "you're irredeemable", etc.
You don't kick someone when they're already down and then say, "gosh well, you don't even wanna get back up do you? You have to really want it?" Why should they want it or try harder when they're expecting more people like you to kick them right back to the floor again. Google "labeling theory", it might be helpful.
There's an therapeutic approach called Motivational Interviewing, and its main tenant is that change is facilitated by and comes from the client alone. It helps a client who is already well aware of people around them telling them to change, or societal expectations, or maybe more simply they have considered personally making changes themselves whether it's something like smoking cessation, substance use /harm reduction, emotional management, or wanting to make other behavioral changes but they haven't because they're not sure what such a change would mean or look like in their lives. So they sit on the fence. They become ambivalent.
The only job the therapist has in this is to help the client lay all this out. To discuss it and weigh each side. To examine what making changes or no changes would mean. Ultimately the goal is to help the client reach a state of active change. So that the client can begin new parts of their life and continue moving forward. But if someone isn't ready, then they're not and that's also ok.
If you immediately shut down on a fictional character because their story wasn't insightful enough for you to glean any empathy that they were capable of change, how do you or will you tolerate the overall change process that real people have? Because it's an arduous one. It takes time, it doesn't sprout up overnight. And half of it is because those people themselves were repeatedly told that they couldn't change either.
So stop contributing to the ambivalence or the stagnancy and start contributing to the change.