"Where did the lit branch come from?"
"I don't know who threw it in. I saw it almost went out and got curious, so I tried..."
"Is it because it almost burned down the warehouse, so you're scared?"
Finding the source of fear is the beginning of healing.
"No... it almost burned someone alive... I ran..."
After Zheng Xu finished speaking, Qian fell into a brief silence. Could it be...
"Do you remember who that person was?"
"I don't remember..."
At this point, the logic was pretty much clear.
Zheng Xu nearly killed someone by setting a fire and had been suppressing it for years. It's a human instinct to forget significant traumatic events. He couldn't remember, so it turned into a psychological issue. Now, finding the unlucky person he feels guilty about would solve the problem.
Zheng Xu didn't remember himself. This shows that person is buried deep in his heart.
