Hear me out. I think you would nail a "being saved" trope. In the sense that one of them is bad/evil or corrupted or just think they're underseving of love for some reason. The "unredeeming" characteristic can vary depending on what you feel like writing, but I think you could write beautifully about depression and how it affects a person's self-worth

What I think about has a "we got so far but in the end it doesn't even matter" twist. You know when the mc is working hard to get better, maybe he stopped doing his evil things or left his bad habits, maybe he's trying hard to let himself be loved and noticing his good qualities, but then it all comes crashing down?

Like all it takes is a big conflict (not necessarily with the other mc, could be third-party induced, but I feel like the other mc undoing all the progress he's helped do would be spicy) for them to slip back to their old habits

The details are better out being left to you, but of course, me being me, I would like a happy ending if you do decide to write it. But I'm sure you'd do justice to an angst ending as well