There's two ways to look at it.
From the game/story perspective. The villainess is a challenge that the heroine has to overcome by winning the hearts and minds of the capture target or targets to defeat her. The 'good end' usually results in the villainess banishment or something worse...
Congratulations you have raised a King Ghidorah. Your reward is crushed to death, incinerated to ashes, shocked to death or all of the above. Have a nice day
You also have to take in account that the difference between free and slave is how much you one at birth and where you was born. In America the dividing line was race
There was also a time that slaves could buy their freedom, that ended in America also