Brainstorming help: Villain motivation

Possibly, one simple thing is that demons are CE, period. Thus, he wants to destroy Good. This might be a bland choice, but I think there's fun to be had in exploring a mentality that is so utterly warped and twisted that its motivations are almost alien. These demons are suffering and destruction incarnate, and to them, increasing the about of Evil and Chaos in the universe is a "good" thing. It prompts the question of "Why do we see Good as good and something to be kept alive?". Making the players and the PCs ask that question might see the PCs reaffirm their beliefs or perhaps turn to join the demon. After all, isn't it just nature to want to be the last one left standing?

A love of his -- possibly someone he fell for in a mortal guise, possibly the tiefling's mother -- was a crusader of the deity of that plane, and was sent forth on a crusade, only to perish. Her soul ascended, and is now out of his reach forever -- or until the gates of that god's realms fall. For an added kicker -- she KNEW he was a demon, and didn't care, or actively sought to redeem him, because she believed he could be more than the creature he was born as.

A fun idea, he's from a lawful good plane of existence (the one he's trying to destroy) but that plane started as a normal neutral plane. He fought for good there and good won but over time lawful good took over power and when there were no more evil or neutral people, suddenly even the neutral good characters like him were to be eliminated. He fled. Here he is. He's not so happy or good anymore.

The planes of existence in standard D&D are balanced against each other and built around the material plane. Using a similar ideology, perhaps this demon’s motivation is oblivion. They theorize that destroying the plane of good will cause the good/evil balance of the multiverse to tip drastically towards evil. He’s trying to destroy good as a cosmological force by destroying the plane. By doing so he eliminates the cosmological balance, and the other planes of existence try to compensate but end up falling into each other, thus destroying reality in a maelstrom of chaos.

Throw in some selfish motivation: The demon is one of four middle-manager-level demons that have been granted this power and told what to do with it. The one that gets the farthest wins promotion to director-level, with dominion over the others. This gives you several baddies, the chance for players to kill some early or to play them off against one another, and a motivation all rolled up together.

What about a lawful evil motive. He faction is at war on a planner scale. His individual motivation is that he is seeking power. He’s trying to take control of the astral plane. He’s inherited parts of a few gods profiles and seeks to throw the others out and rule as one. With this you have the option of a grand near max level boss fight. Additionally if he’s part of a hierarchy that allows for endless multi level boss fights for your pcs. In 5e classic setting the gods are barred from directly intervening in mortal affairs this is why s Demi god who can access the material plan has all the power. It also gives your players opportunity to make allies with the opposing factions minions.

Also gives you the option to allow your players to overthrow him through deception and subterfuge.

Keep in mind one fact that if you have an enemy that directly relates to a single player it’s best to engage allies or enemies of other players as well this keeps with a good level of immersion while not promoting a single party member to the roll of party leader and possibly alienating other players.

A magic user pulled the son into the plane, father wants to destroy all of the plane leaving this magic user to suffer alone in the plane for eternity.

Gives some backstory to some powerful magic user and a character background development as well.

[What's your villain's motivation?] (http://www.creightonbroadhurst.com/gm-advice-whats-your-villains-motivation/)

I hope folks won't mind a semi-rude answer...

When I was in graduate school, a good friend of mine did his research on Elizabethan rhetoric. (At the same time, I was looking at political rhetoric from the American Revolution.) He admitted that he saw no practical use to his research. He could not imagine how it might help him be a better teacher or change rhetorical theory in any significant way. In the end, he summed it up by saying...

"You know why I do this research? Because it makes my dick hard."

If your bad guy is a demon, maybe that's the only motivation he needs. For anything.

I hope this helps.

Two thoughts. Either might work.

Idea one. Spite. He was once defeated by a powerful magic user who awakened something to do the deed. So now he's going to use awakening to destroy that magic user's entire plane.

Idea two. Prophesy. The demon has been prophesied to be killed by someone from that plane, but rather than go through all the bother of searching that someone out, he's elected to nuke the entire thing. If the party manages to kill him, whoever lands the killing blow is retroactively the one that was fated to do it.