From what very VERY little context a I've gleamed, since the plot is basically microscopic at this point....
Sangwoo's father (shitty ML's father) was highly abusive to him as a child. He later met the bottom and fell in love, faking being nice and playing happy family together. The bottom fell for the act and fell in love. Sangwoo fixated on the bottom because it was the first taste of kindness/normalcy he ever got in life. He's determined to "steal" him away from his father and have him solely devoted to him. That's what this is all about. A twisted story of revenge on someone already dead and supposed love that's really just fucked up obsession and misplaced feelings.
But really, it's just t*rture p*rn.

I don't understand so the husband was the bottom and he was cheating on the guy or what is that photo?