Satan Is Good
You know the story: a long time ago, there was a conflict between the Powers That Be. The good guys won, and the bad guys got to suffer (a popular misconception would be to rule) in Hell instead. But it could be you're not getting the whole story. Maybe Old Scratch (or his in-world counterpart, thinly-veiled or not) has been Wrongly Accused. Maybe his loss in the war with God/the gods was the original Downer Ending, and the bastards who did win have been tarring him with the propaganda brush ever since. Wouldn't that be a Mind Screw?
Another variation is that God isn't necessarily evil; he and Satan may just have Creative Differences. Or perhaps Satan is willing to play a Zero-Approval Gambit and be a Dark Shepherd to help keep people in line, the "stick" to God's "carrot". Or maybe God just doesn't care, or, similarly, he vanished from the universe sometime after making it, and the other angels are evil. It's even possible that he and Satan are actually good buddies. In these versions, if the reasons behind it are mentioned at all, Satan's bad reputation may be chalked up to misinterpretation on the part of mankind; particularly if he and God don't really have anything against each other. It could even be that the entire "warring angels" part never really happened. "Demonizing" the, well, demons could be the work of a Corrupt Church.
This was quite common in earlier times, where Satan was seen as a servant of God, who performed an important role in testing peoples' resolve. Satan is derived from the Hebrew "ha-Satan", which quite literally means "the Accuser" — and what he accuses you of is sin.
Satan Is Good - TV Tropes
'Satan Is A Nice Guy; He's Misunderstood': Miley Cyrus

You know the story: a long time ago, there was a conflict between the Powers That Be. The good guys won, and the bad guys got to suffer (a popular misconception would be to rule) in Hell instead. But it could be you're not getting the whole story. Maybe Old Scratch (or his in-world counterpart, thinly-veiled or not) has been Wrongly Accused. Maybe his loss in the war with God/the gods was the original Downer Ending, and the bastards who did win have been tarring him with the propaganda brush ever since. Wouldn't that be a Mind Screw?
Another variation is that God isn't necessarily evil; he and Satan may just have Creative Differences. Or perhaps Satan is willing to play a Zero-Approval Gambit and be a Dark Shepherd to help keep people in line, the "stick" to God's "carrot". Or maybe God just doesn't care, or, similarly, he vanished from the universe sometime after making it, and the other angels are evil. It's even possible that he and Satan are actually good buddies. In these versions, if the reasons behind it are mentioned at all, Satan's bad reputation may be chalked up to misinterpretation on the part of mankind; particularly if he and God don't really have anything against each other. It could even be that the entire "warring angels" part never really happened. "Demonizing" the, well, demons could be the work of a Corrupt Church.
This was quite common in earlier times, where Satan was seen as a servant of God, who performed an important role in testing peoples' resolve. Satan is derived from the Hebrew "ha-Satan", which quite literally means "the Accuser" — and what he accuses you of is sin.
Satan Is Good - TV Tropes
'Satan Is A Nice Guy; He's Misunderstood': Miley Cyrus