I'm pretty sure the canon reason that Nye works for Mevolent is because Mev lets it do what it wants for its messed-up experiments as long as it in some way, shape, or form benefits Mev and his cause.
BUT....
But I read someone's headcanon (dunno who anymore, sorry) that Serpine is the one that cut off Nye's nose and sew shut its eyes and mouth. Mev wouldn't allow Nef to attack another creature in his service and with that under his protection, unless perhaps Nye did something to displease Mev causing him to unleash Nef on it.
But what if...
...Nye is the only Crenga Serpine managed to capture, and this rare creature was a special treat for his sadistic appetites, and he was just tearing it apart using it as his study subject to find any weaknesses he could use against General Mantis, another crenga that unfortunately had just decided to become a problem for Nef in the name of the sanctuary. He sewed Nye's mouth shut when its shrill voice started to annoy him and sewed the eyes shut while whispering into its ears how pathetic that pleading look in them was, as if he didn't enjoy it.
But because Crenga are so rare, Nye's capture drew Mevolent's attention, and he ordered Serpine to cut the strings keeping its mouth shut so he could hear its case. Nye managed to convince Mevolent of its usefulness and ordered Serpine to release it into his service. Nef was remarkably pissed about losing his exotic toy, so he cut off Nye's nose in spite.
"He told me to hand you over alive and functional. He never said anything about 'in one piece'."
So the creature would always remember him and the 'quality time' they spend together.
And that's why Nye is loyal to Mevolent for the most part. Because it got thrown into a pit of vipers where one half, including Serpine, sees its cowardice and lack of fighting abilities. Weakness is a provocation to mages like them. While the other half, including Baron, dislike it for the nature of its experiments and would like to put a permanent end to it by taking its head. Naturally, both sides have quite a lot of people that see creatures, even humanoid ones, as lesser than mages and treat it accordingly.
And Nef has it out for it specifically bc his toy turned into a rival, and he didn't like that. Not at all.
Mev was standing between Nye and annihilation. And once Mev died, Nye sold all of them fuckers out to the Sanctuaries so quickly, you would not believe it. Imagine Nye being the one that ratted out the Serpine was planning to hide with the necromancers in America and being quite disappointed that the Dead Men didn't mange to kill him lmao Nef so tried to get the oversized critter for that, but the Banshee/Dullahan wouldn't help him get to the warehouse where Nye was hiding and Nef was so pissed-
