it's less hypocritical than that. in-ah knows the rumors of what the crown prince has been doing. if it were someone less malicious, he might be considerably less upset, but with the prince's cruelty and regarding of people as disposable, in-ah's reaction is more like someone finding out their loved one is being hunted by a ravenous beast.
Thank you for sharing your insight! Sometimes reading the chapters as they're published I lose the details of the story. So I thought the ML didn't know of the crown prince "indiscretions" since he didn’t mention them while talking with his bearded friend and told him to find out what the crown prince is scheming. Maybe I need to re-read from the beginning!

Our ML In-ah is suspicious and angry at the crown prince for his vulgar and filthy eyes toward his brother-in-law when, from the start of this story, he himself has fantasized and even spent nights polishing the wood thinking about the same brother-in-law!
(with this I'm not saying that the crown prince is not a piece of garbage for the abuse towards his servants but that the ML shouldn't condemn him for impure thoughts when he's doing the same!)