I have to agree; though it began as consensual it later became forced. Same thing with everything else the evil ones "made a deal" for, since we're told also that when people could no longer provide enough food to satisfy them, they began to eat the people. This wouldn't impact human responsibility though, since humans had to have agreed to work with the evil ones in the first place. But like Darth Vader, "We are altering the deal; pray we don't alter it any further!"