I believe it was because he sees Cain as weak (not physically but behavioral) compared to Satan, so now he's on Satan's side because of that. Which is just straight up stupid.
I'm also seeing people say it's due to his inner demons, or whatever they are.
Yeah that’s what I got. Sid is conflicted. His ‘core self’ was always loyal to Cain, but his inner demon is super dark. So the two sides of himself would fight, we’d see ‘inner Sid’ arguing and fighting his demon-side (the mass with no body and all the eyes), but inner Sid eventually was overwhelmed by his inner demon. That’s when he betrayed Cain.
Ohhhh THANK U GUYSSSS
Wait I’m actually confused what is sid trying to do again? He started out being cains friend n then suddenly turned on him??? I wasn’t really paying attention to sid can someone explain