I kinda suspect that Crowley, knowing that he had no choice once turned and that Ferid is stronger and more devious than him, chose to act according to the old wise truth of “if you can’t beat them, join them” and has been bidding his time to deliver a deadly blow to Ferid if Ferid ever shows any opening to him. I guess props to Ferid for never giving him such a change for all of 800 years, because it goes without saying that Ferid knows that Crowley won’t hesitate in the slightest if ever given the opportunity, but he, too, took some of the old wisdom to heart, namely “keep your friends close but your enemies closer”. That’s why there is that (very obvious, yes) passive aggressiveness on Crowley’s part, and that commanding tone and thinly veiled threats on Ferid’s part that make it to the surface from time to time, like in the audio drama with the two of them (“I’m asking questions here”, “Answer within 3 seconds”) and in recent chapter where Ferid forced Crowley to chase the Shinoa squad, all just beneath the veneer of the superficial banter (that seems friendly and good-natured only if you really don’t know any better).
I’m sure that’s exactly one part of the dynamic.
The other part I’m sure is also how Ferid manage to snag him in the first place, and preyed viciously on Crowley’s moment of weakness.
Crowley is intelligent. If anything, I feel he could probably go toe to toe with Ferid as far as the wits go. After all, he learned from the best. But, even still, he knows that Ferid has him outmatched, especially in regards to pure strength. Crowley is powerful. But Ferid is not only older, but higher in rank, and listed as being the second most powerful vampire in Japan. Crowley is smart. He’s biding his time and holding back for a better opportunity. Plus, at this point, there’s a little bit more to worry about. There’s no sense in causing yet another divide. They kind of need each other at their back. Just like with the first light novel, when he finally came around to accepting Ferid’s help. Plus, he’s waited this long. Though he has no doubt that Ferid is going to use him as another scapegoat (which is also why I think we don’t see Chess and Horn nearby. Lets face it, those three are attached at the hips.)
I’d also be willing to bet there’s a thin layer of Stockholm Syndrome at it’s finest, even if it’s small.
Ferid saved him all that time ago. Crowley still has a very strong sense of honor, and that also buys into it. He feels indebted to Ferid in a way. But also doesn’t think he’s much better, himself. And when you live with years of emotional and mental abuse, well, you kinda start believing it yourself, especially when you are at such a low point. So there’s this building hatred, but it’s almost parasitic of like, what would I do if it wasn’t there anymore?