Why would people be enslaved? and why would someone enslave another?

Answer: Many reasons. Most of them terrible.

But possibly for the identical reason that a decent parent who is totally against penalty would be willing to do and use it if their child keeps mindlessly walking into oncoming traffic (regardless of what number times they tried other methods to prevent them from doing it).

Sometimes you'll be able to keep your principals and morals ... and stick out the dead kid. otherwise you can do whatever is important to prevent them from killing themselves and others ... whether or not it means becoming a tyrant and a dictator in their eyes and therefore the eyes of others. whether or not it means going against your own core beliefs.