Are you for real ?
Nanjo beat the shit out of Mokyeon because he outed him during their threesome with Sunhwa, which was the thing he was most afraid of ! And then to prevent him from going after Sunhwa to have a chance to explain, MK fucked him so deep he forced him to pass out. Yeah, those 2 had a deal, as messed up as it was, but Mokyeon threw everything out of the window to selfishly satisfy his own desires and obsession, with no regards for what Nanjo wanted. And to be honest Nanjo was really patient, Mokyeon could have been decked way earlier (for example when he almost made Sunhwa suffocate under the pillow out of jealousy during another threesome).
Punching someone in retaliation for the fucked up things they did to you is nowhere near equivalent to being "abusive". What kind of double standard are you trying to insinuate here ??

Gotta love how Nanjo accused Mokyeon of being an abusive boyfriend, only for Nanjo to be the first one to beat the sh#t out of someone.