The premise of "Devilman" is simple - defeat demons by becoming one. The 70's popular animation was adaptated from the "Devilman" comic book series which chronicled a simple school boy who becomes a part-time "Devil" and fights against demon clans about to take over the earth.
Rooting for the "Devil" sounds odd now, but as a kid, I didn't care. I just wanted to see Devilman kicking other demons' asses.
I suppose it's similar to the show "Dexter" where we find ourselves rooting for the serial killer.