I think Musical!Betelgeuse and Movie!Bj are different characters in the way they are being portrayed at first
Musical!Bj is desperate. He wants to be in the living world so bad. His purpose/goal gets established quite early. And his only “door” to do so are the Maitlands, and they turn up to be really boring. And then he meets Lydia, who can seem him and he has to make a new escape. Yes. He’s creepy but the musical gives it like, a comedic thing to it. But then he gets stabbed in the back. It doesn't feel like a win to the viewer but some kind of betrayal
The musical also relays in that people have watched the movie before, specially for some jokes.
Now, let's with Movie!Bj (this is about the first movie, I'll make a post about the second one some other time) From moment 1, he's meant to be...bad, yeah that's the word. He looks like a cadaver, he has moss growing on his face, a constant reminder that he’s not alive. When he offers a deal to the Maitlands but we don't know if he has been stalking them. Lydia's encounter with him feels orchestrated and so the deal and getting married to her. Barbara riding the sandworm that then eats him is viewed as poetic justice. It's not after a bit of thinking outside the movie, that you start thinking that maybe, just maybe, he had a slight tiny super tiny bit reason to do the things he did.
From a meta pov, What conclusion do I get from this stupid rambling? As I'm not a critic nor I have any major in theather or cinema, I understand that the years that passed between the musical and movie, there was a shift on writing the archetype of ""likeable""" villains you're not """supposed""" to like