J gets too much hate and his uncles too much love
While rewatching, it's even more apparent to me how little J's uncles care about him. They treat him worse than Lina, not only giving zero fucks about their nephew, but treating him blatantly as a threat instead of a person. It wasn't until season 5 when they started giving him respect, and that was only because they finally realized how badly they'd be screwed without him running things. Throughout the series, they treat him as a co-worker - an underling to boss around - never as family. Only Pope showed him a modicum of affection, and even then he'd immediately switch to being cold to him afterward. Love is continuous, not sporadic.
So, you wonder why J turned on them in the end? He showed them they were just as expendable as they thought he was, that he was truly faking and burying his emotions almost every time they were assholes to him. His mask successfully hid the hatred accumulating within him for his uncles throughout the series, and while they didn't see it, we were constantly shown scenes of J glaring at them whenever a coversation ended. J is a sociopath, but he still could feel love and show vulnerability as shown early on with Nicky. Smurf and his uncles killed that. They always had a chance, and they never took it.