Yeah I think they never really gave Bela a chance. She had a good intro episode, we got a lot more of her in her next episode including hints of her having killed her family (an episode Kripke and co now complain about having too much Bela focus...), but then the rest of her appearances were basically just cameo's for the rest of the season, until her last episode which flash-backed to her reason for making the deal before she died. With both and her Dean having sold their souls and being marked for death, I think they could have made a lot more of that, maybe through contrasting Bela being willing to do anything to save herself in comparison to Dean and his having certain lines he wouldn't cross with the immortal Doctor for example. Instead they didn't even tell us that she made the same deal as Dean until her last episode

But then I think the same thing about Jo. She had her intro episode and a lot more screentime in No Exit, and then the writers basically gave up on her. I don't understand why they don't respond to hostile fan reaction by developing those characters more and giving viewers reason to love them, that's what Joss would have done. SPN writers seemed to basically give up after a couple of episodes and then blame the fans for not accepting female characters, even though often audiences are iffy with any new character at first until they get to know them better and writers do more with them. Jo's return in season 5 showed that many fans would have been open to her if the writers have given her more of a chance. For all the fuss made about Jo, Ellen, and the Roadhouse in season 2, they really only played a very small part before Kripke and the writers decided that it wasn't working out