So, for these Monster-Lunars who are set up so the easy path is to be a NPC-madman but where a PC can be playable with a group, I think a good starting point would be a full integration of Limit as an active resource. Taking the term "Limit Break" back to its Final Fantasy roots (and also touching on the original werewolf stuff, not Apocalypse werewolf stuff [1]), it's a source of power for them. You build up Limit which makes you crazier and eviller, and then you vent your crazy-evil to fuel effects. Your war form is something you access by going into Limit Break.
Just retouching this point, hmm. I don't think you actually want to spend Limit to activate powers. Giving into your evil side and letting its power out shouldn't make your evil side weaker. So at the very least, your Limit should only decrease at the end of the scene, or possibly next time you wake up (for that whole 'evil rampage, pass out, wake up sane again' thing). You might even want a divide between Transitory Limit and Longer-Term Limit (because the term "Permanent Limit" isn't really appropriate for a splat who has their mechanical integrated around raising and lowering Limit), so you can raise your Limit by letting your evil side out for a little while for a combat, which gives you points of Transitory Limit and which also raises your Longer Term Limit.
Limit Break War Form is still totally sweet as an idea, though. Gotta have your 'letting out the monster' form.
So, this set-up doesn't seem very friendly for all-Lunar parties. Most Lunars are NPC lords of Chaos, ruling over the Wyld proximate to Creation (we can use this chance to utterly clear out and rework the Wyld while we're at it, like bringing in @Havocfett's Rakashasa). Even compared to an Abyssal Loyalist game, it doesn't sound very friendly to play to me, because - barring chaos-prince Lunar elders being a thing - getting a party of madmen-type Lunars pointed in the same direction sounds hard. So the splat should really be built up to enable the story of the kind of character who's trying to resist the evil power within - werewolves, the Hulk, Jackie Estacado, etc.
Sounds to me therefore that one of the big ways they have for venting Limit and keeping their evil side under control is positive connections to other people - like, say, other PCs. It'll be the opposite of an Abyssal Renegade thing. While Abyssals have to keep people at arm's length to avoid their curse hurting people, these Monster-Lunars have to find things, people and ideals to love and cherish and admire that they can use as a bulwark against their own madness. Their loved ones can plead with them when they're running full Limit to lower it, allowing sanity to take hold again. Someone can stroke their face and tell them that the woman they love is still in there.
And someone with a lot of friends and who values the cause they're fighting for? They can look an enemy in the eye and whisper "I'm always angry".