Female demons loved 'bad boy' demons. This was how mate selection worked in the Abyss. Although Julia was a fallen angel, she was still from the Abyss, so she was no different from other demons.
The Lord of the Hollows would never have thought that Roy, a high-rank demon, would dare to agree casually to the condition proposed by a demon lord. However, he immediately regretted it.
Perhaps it was because he had been in this world for too long and had been in the Charred Council for too long, causing the Lord of the Hollows to forget how to deal with a demon: all verbal promises without contractual restraints did not count!
It was not that the power of the Lord of the Hollows was not attractive to Roy, but Roy felt that there was a pit in this job and did not want to jump in!