To really be able to feel that something is good, they had to create something really really bad to contrast it.
Making it easier for the people to see black and white and follow that they thought was the right path.
If Jesus and Satan looked the same and had similar views, people would be conflicted and they couldnt convert so many people back in the day to christianity.
With "the other side" being a hulking monster, it was easier.
Never met the fellow myself so cannot say if its an accurate portrayal.
Marty knows i'm sure.
