From hereon out, I'll tag this kind of thing 'fluff'. This sort of thing never comes up directly in a game, but its a "headcanon" that informs the game.
Chaos isn’t a party. It isn’t unbounded freedom (at least not for long).
It’s the ground state of creation. It’s a bonfire followed by a heap of ash. It’s the last photon in the universe decaying, then an infinity of darkness.
A forest seething with life and filled with predators and prey is order. An empty, crumbling parking lot is chaos.
 |
| (Zdzislaw Beksinski) |
A town filled with looping side streets and poorly marked cul de sacs might feel "chaotic" in the colloquial sense (at least if you’re a stranger to the place). But a district of tightly gridded, identical apartment high rises with identical units filled with identical furniture is an environment where chaos reigns in the cosmic sense.
 |
| On the right: Chaos (Photo by Ursus Wherli) |
Anyways, you see what I’m getting at.
Demons are chaos, forced by foolish mortals into a fleeting prison of flesh, because pure Chaos simply can't exist in our universe of ordered matter and energy.
Demons don’t hate life and order; that would require something akin to passion. It’s just that everything that exists (including themselves) causes them pain and misery—the kind that human nervous systems don’t have the hardware to comprehend.
They endure their own miserable existence only because they have no means to end it, and once they’ve been summoned, they seek any and all means to hasten the return of the universe to its normal state. During their tortured birth into the material realm, a horrific arrangement of organic forms, concepts, and nightmares-made-real cling to them like dust clinging to a bead of water.
To decide what that looks like and what it does, you could use Appendix D at the back of the 1e DMG. Or make it a Paradox Beast. I tend to just stick whatever traits I like on board. They haven't come up often for me to come up with a unique thing for them mechanically.
With regards to PC alignments...
Neutrality is just near term pragmatism.
Lawfulness describes, simply, a spectrum of preferences for order—even a tyrannical or capricious order—over chaos.
If you want some ideas on how a truly Chaotic NPC looks at the world, look no further than Thomas Ligotti.
When asked by an interviewer what would make him smile, Ligotti replies:
“I think that if I could walk from one end of the world to the other and see nothing but annihilated landscapes, that would make me smile.” (source)
 |
| Warhammer Chaos Warrior (it's probably sacrilege but I don't know the illustrator! Let me know in the comments and I'll add a proper credit) |
