Merrick takes a shine to Joe because he has spunk. The headbutting really did it for him, and stabbing Joe right after and watching him bleed gave him a big ol' hard-on. He becomes obsessed with getting Joe to hurt him more so he can get off on it sexually, non-consensually. Maybe Joe takes the bait at first because hey, he does want to beat Merrick's face in, but once he realizes Merrick his turned on by it he feels disgusting. It escalates, and Merrick wants to make it all the way sexual--wants to blow Joe, rim him, stab him and use his blood as lube, say gross kind of racially-charged things about his big brown cock, etc. etc. Joe is trapped in this weird horrible nightmare because 1) he does actually want to hit Merrick but 2) not like that and 3) doesn't want Merrick anywhere near his dick but also 4) doesn't want Merrick anywhere near Nicky, which he threatens whenever Joe doesn't do what he says. His sense of agency and what consent even means gets seriously fucked with because technically if he's beating up Merrick doesn't that mean he's a willing participant??

+ bonus for Nicky's reactions
++ extra bonus for hurt/comfort or dealing with the feelings after it's all over