And after his manhood was ripped off, the immense pain freed the male cultist from the [mental suggestion].
This demonstrated that intense pain or other external stimuli could break mental control.
When the other cultists entered the room, he seized the chaos to issue mental suggestions to the other two non-Evil Spirit Cult members, making them believe they were lovers. They fell for it easily, likely because they did not possess any gear to resist mental attacks, which was extremely rare.
Next, Zhou Yuan anticipated that the Evil Spirit Cult members would use an AOE skill. This would allow them to crudely and directly locate the enemy's position and also stop their mad teammates, but the cost was that all teammates would suffer mental damage and become idiots.