Sleepwalking?

Wang Haiming was possessed by something inside the mental hospital, and the thing followed him out when he exited the hospital. Both parties were constantly fighting over the control of his body. Wang Haiming decided to die rather than compromise.

The man's weird death and the actions that happened at midnight supported Chen Ge's speculation.

After Wang Haiming's death, the thing he carried out with him from the mental hospital probably stayed back in this room. When his body was carried away, there were plenty of police and spectators, so the monster definitely did not show itself. When night came, Room 303 was sealed, and the monster had no chance to go look for a new target.