yes, it was explained in the show i believe, though it wasn’t greatly elaborated upon
now i haven’t seen earlier seasons for a while now and so please correct me if i’m wrong at all here and i’m going to just kind of try to summarize
Bela was very young when she made the demon deal which resulted in her parents’ deaths.
Her father was abusive, physically and sexually. He did horrible things to her and her mom didn’t ever try to stop it, hell I don’t even remember if they really mentioned her mom??
She just wanted to get away and stop being abused and hurt and molested and possibly raped. And then a little girl, possessed by a demon, showed up at a playground to Bela and told her that she could make the abuse go away and stop her parents from hurting her. She told Bela that it was easy and that it wouldn’t cost her anything that really mattered.
Bela, of course looking for any chance to get away from this, happily agreed. But she didn’t know it would result in her parents’ death, and ultimately her own.
She wasn’t aware of what she was really doing or what the demon deal meant/would do/would cost.
She was terrified at the end, and you know, it really makes me angry that they killed her off because of fans having a rabid hatred for any recurring female role on the show. Sam and Dean could have at least tried to save her…..Oh well.
I have a lot of Bela feelings and I love her a lot. Hope this was at least somewhat helpful