Monarchy
Looking at Erza's enthusiastic look, Venn smiled. As the official lord over the mine, he needed someone to assist him and the only suitable person he could use was Erza, the lesser demon.
The Lamia guards that were left behind by Rena could not be used as their loyalty ultimately lies with Rena. His actions in the mine would only be monitored by them and reported to Rena's ears. Thus, it was best if he could involve them as little as possible.
The only other resources he could tap on were the recently freed succubi and the docile human slaves in the mine, however he had to gain their loyalty first.
But if there was one thing that Venn did not believe in - that would be the concept of loyalty.