The villain certainly becomes strong and has enough ability to kill and cause a lot of trouble, so his ability was certainly good.
If he was still human and wasn't full of revenge like in the novel, then Luciel would certainly invite him.
In the novel, the villain was already only full of revenge and didn't care about anything at all as long as he could achieve what he wanted.
It was also the same with his subordinates; they were really like-minded people in the novel, and their goal was very clear.
So even though a lot of them died, no one would care about the deaths of the people in their group as long as they achieved what they wanted.
They all have somewhat good abilities, especially the villain, who was the strongest in the group.
So, it was really good to recruit the villain if it's still possible.
It's good to have more skilled people around, but if the villain turned out to be that bad already, then Luciel would just kill him.
