Kars

Lucifer used some kind of illusion to break down Rouge's mental fortitude and was making sure that he wouldn't be able to say no when he asks him to help him.

After all, even though the leaders of the Warmester Denoble House, especially the Patriarch, are dead now, that doesn't mean that Lucifer has taken control of the Warmester Denoble House.

In fact, if the Warmester Denoble House learns that Lucifer killed the Patriarch and the 6 pillars, the most natural course that would take place is the Warmester Denoble House seeking revenge against Lucifer, who killed their leaders.

That was why Lucifer needed someone like Rouge, who can take the place of the Patriarch, and govern the Warmester Denoble House under his rule while becoming a subordinate for Lucifer.

Although Lucifer didn't really plan to spare Rouge, there is no other choice right now as the assassination was successful.