It's unbelievable, he actually chained him as if he were a dog. The puppy play always gave me a bad feeling, because you could tell that it wasn't just a play to Taeseung. Several times he expressed his desire to lock him up and now he literally did it. I'm worried about how being confined it's gonna affect Sooyoung's already unstable mental state.
It's interesting because the puppy play wasn't introduced just as a random thing or to add a cute little detail to their relationship, it was used to mirror their dynamic outside the bedroom as well. Taeseung wouldn't let him leave the house without someone 'holding the leash' and for a while he would stay home doing nothing, just waiting for Taeseung to come home. Besides, Taeseung has been dehumanizing him for quite some time, he referred to him as a toy a couple of times when talking to others and even in his inner thoughts, but chaining him to the bed is another level of fucked up.
No matter the reason, you can't chain a person up. It's dehumanizing. There's no excuse for doing something like that. You're depriving a person of a basic human right, their freedom.