It’s just my theory. I thought of it coz he was trembling and shaking when he saw the red haired guy at the cafe like a major PTSD hit him. Also both he and Taesung has gone through so much before, so it can’t be something as simple as a normal “stay away or we’ll hurt you/Taeusung” threat for him not to return to his partner again. So I assumed it has to be something really horrible happened to him that is far too much for him to face Taesung again. Also the red haired guy said “after what happened to him, I didn’t think he’d function at all”, so it has to be something very traumatic.

I think Haebom was kidnapped probably used as a sex slave, assaulted and tortured, hence why he refused to go back to Taesung in the first place. The guilt hit him hard. He probably feel he is tainted and disgusting. But I hope he tells Taesung the truth so they can go through this together and settle it fast.