Just a small question,what would push john over the edge to go grimdark?

I don’t quite think John can get even close to Grimdark. We see him get mad at Jack and Caliborn but like. The fight with Caliborn was doofy hand-slapping fisticuffs, and the fight with Jack in the Bubbles had him giggling after his Hammer put a hat on Jack. You’d have to physically find a way to corrupt him a-la Crockercorp Jane to get anything Grimdark out of him.