These are the same savage bastards that hunted natives with war dogs and laughed when the man, woman or child was caught and torn to shreds, the same guys that raped their way across the continent; yeah good thing they stopped those natives from ritually killing people. They definitely liked to enslave the local populations wherever they went but they were bad managers of their human resources, killing, beating and starving their slaves to the point that the natives were largely eliminated as a viable work force. Good thing there were black people in Africa just waiting to become slaves and they were more resistant to European diseases, lucky for them they cost enough gold that they weren't slaughtered indiscriminately for the most part.

Also most conquerors try to assimilate or stamp out any completing power structure, especially competing religions.