I pretty much expressed my thoughts on the matter in my previous answers (here and some here).
To summarize: revenge is part of it, yes, but Crowley was established to be smarter than that: he prioritizes the information about the bigger picture, about the grand-scale plan that Ferid has underway and for which he apparently needs Crowley, so much that he even marked Crowley 150 years before Crowley was born and went through the trouble of stalking Crowley basically all his life and eventually turning him into a vampire with the blood of an insanely high ranking progenitor. You see the same approach that Yuu took in the latest chapter where he agreed to forget what Ferid did to his family, prioritizing getting information out of him first. Except to Crowley, revenge itself over time turned into something more like a purpose than the burning feeling he had in the beginning. While he’s at it, he’s trying to find upsides in his situation and alleviate his boredom via tagging along with Ferid and his crazy ideas, because why the heck not when he has to wait literally for ages for the situation to change and give him a chance to make a move on the way towards his goals.