You didn't watch "Manderlay", did you? The blacks, especially Wilhelm and Timothy, basically play Grace like a fiddle in the end because, her good intentions aside, she's trying to do these things to assuage her guilt over her skin color and privilege without truly thinking of them, the outside world and the consequences. She doesn't realize they don't "truly" need her as she really is until it's too late; in the end, they want to put her in the same place she was among the people of Dogville, a "slave" to the hierarchy of Manderlay. Unlike being the slave she was there, she's forcibly enslaved to a position of "power" because of what society sees of its minority citizens.