Actually in reality juyeon wasn't originally sadistic, he didn't want to hurt jikwon but cuz he angry that jikwon basically forgot about him and what he did to him so out angry he doing all that but in reality in begining when jikwon first got hurt on his head JY went to visit him in hospital and when he knew he forgot his memories he wanted to start over with jikwon as if nothing happened but jikwon wanted to rape him again
And toward the end when Jikwon became scared jikwon would abandon him, he will try self harm to get jiyeon sympathy, JY would be so mad when JK do that, so JY doesn't want JK to be hurt, if he was real sadistic he wouldn't care what happen to JK
Broo this story has sm potiential to be a psychological thriller instead of a yaoi like w/o the rape if they twisted it in a way where juyeon js went the like mental fuckury route, while the jiwook has no idea why and then the big reveal but noo it’s js rpe