Illusion

Lucifer agreed to heal Rouge when Rouge asked him to. After all, Rouge wouldn't be able to do his services if he was about to die literally.

The only reason why Rouge wasn't screaming in pain or whining in pain is because of his willpower and out of fear for Lucifer.

After all, Rouge knew that he needed to prove his worth so that he would be able to live.

No...Rouge knew that he needed to please Lucifer so that he wouldn't experience the same nightmare that Lucifer put him to when he was still an enemy.

If Rouge would have to go through that again, then...Rouge would surely go insane.

In fact, Rouge would rather die than experience something like that.

And since Lucifer certainly had the power to make Rouge experience something like that again and again, Rouge knew better than to try and defy Lucifer only to experience something like that again.