The nature of Satan is not something Christianity actually scripturally has anything to say about; contemporary evangelicals don't seem to know that they are largely riffing off of Milton. But if you buy Milton, the angels had free will, sure--the fallen ones made a decision and it was the wrong one and they refused to recant it. But the Bad Place demons in the show don't seem to me to be entirely those kinds of beings--and yet they do seem to have desires, personalities, distinctive natures. And free will, of some sort.
I never thought that was possible for a born evil Demon. I forget. Do angels and demons have free will or is that just a human thing.
If you subscribe to the "demons are fallen angels" story, then clearly they had free will at at least one point. I think theologians are divided on whether they have free will now; the popular theory seems to be that then the universe was still being formed, the angels had free will, and some of them decided to leave, but having made that choice they don't get to revisit it; the angels in Heaven are too wrapped up in eternal goodness to be capable of wanting to rebel, and the demons in Hell are likewise stuck with their choice because they're too mired in sin to be able to conceive of repentance. The same is true of human souls after death; once you reach either Heaven or Hell the experience of being there makes it fundamentally impossible for you to choose to be in the other place.
The entire premise of the Good Place seems to be about upending that. Certainly prior to the events of the show it doesn't seem that there was any precedent for humans to find redemption after death, because once you die you stop accruing points, so the implication is that the "point system" follows the same general rules of "free will" in terms of only getting to take advantage of it while you're alive on Earth, which would also mean that angels and demons aren't affected by it. But then Michael does his experiment and all that goes out the window.
"I have not actually recommended many games, and I'll go on the record here saying my track record is probably best in the industry." - schild
If you watched the Lucifer TV series you'd see them doing a riff on that as well that applies to angels and demons both. In that show angels and demons get what they think they deserve and God has nothing to do with it.