I put a spoiler warning up ahead because we're talking about things lol.
I think for Billy the key moment of knowing that was when Curtis gave him kind of an out, asking if he just got turned around and lost, and Billy was like lol no. Because he knows he was always a bad person. It's like that B-99 gif "cool motive, still murder." Then again I think about that for Frank too, lol, which I'm alone in the fandom for.
Netflix has done pretty well in villain department, more so than any of the movies, but they also had time to build them. I think Billy was very compelling, but it doesn't change that he is flat out evil. Even if he did care for some people, he also would kill them in an instant if he felt like it, which obviously he did attempt to do lol.