When I said they were captured I pictured it as all the captives up against a wall and all the guards in body armor with automatic weapons. You know, typical guards?
This evil mastermind would put you on a mission to further increase his power and enslave millions, yada, yada...The Ally but not Evil option: "Counteroffer, you let them all go, and I will lend my great skills to your cause to do anything for you that I do not find repugnant"
Are you sure this is even a viable option? Presumably he will have some means to hold you to the deal.