Why did humans create beings with free will to solve their problems for them?

[ he knows that humanity has a history of slavery, but why bother with a non-human alternative if the moral implications don't matter? or perhaps they really were that depraved. as much as he loves humanity, he knows their capacity for cruelty.

castiel lifts his head and looks around. ]

I don't see any sign of any human here. There aren't even any spirits. I think... I think something must have happened.