I don't think Sangwoo is sad or regretful when he says he'll just hurt Bum again and again...

I don't think Sangwoo is sad or regretful when he says he'll just hurt Bum again and again. I think he is just being honest, and he is a little relieved that Bum doesn't hear and doesn't pick up on it. Normally, Sangwoo is very nice to his victims, leading them on, and then showing his true self while torturing and killing them. With Bum, it is different. Bum sought out Sangwoo, and has become somehow family, graduating to an ongoing cycle of abuse (unless Sangwoo decides to kill Bum too, but Sangwoo doesn't really want to end his fun). So Sangwoo sometimes teases Bum, is nice to Bum, and then *smack* puts Bum in his place. In this chapter, he is always gently teasing this dynamic. In the tub he sist with his back to the smaller Bum, no eye contact, and his hand on Bums legs, testing to see if he can hurt Bum, pulling up the legs to dunk Bum's head under the water when displeased. Sangwoo takes Bum to the theme park, but he teases him, giving and then taking, playing with Bum's feelings, waiting for Bum to scream for him. Sangwoo enjoys the scream too--when Bum screams that is. Rest assured , Sangwoo will continue to make Bum scream.