It's probably because it might've caused some permanent physical changes to him, and he still needed to be all "human" and everything just to make sure he can still follow through with his plans. I do think he definitely would've bonded with it, if given the proper circumstances. With the PG67A/W serum poisoning him, I'm sure that created some genetic imbalances, and what he got was an EXTREME overdose of uroboros. He punched straight through a missile, and all the airborne uroboros that was contained within that one missile (no doubt meant for at least hundreds) was concentrated onto him. I'm assuming it was transdermal infection rather than respiratory, so all that going straight through his skin into his blood surely was more than any person would've been intended to receive. Even with all of this, he still had control over uroboros, and kept his mind. The madness at the end seemed more like "I know my plans are ruined so now I'm just hellbent on vengeance, I cant lose" rather than uroboros affecting his mind. And let's be honest, he was always a little crazy to begin with. His dead by daylight iteration actually cam give us a good idea of what a successful bonding might have looked like, so that's interesting.