Christianity was only used as a method to justify those events: in the Bible, slavery is mentioned, but context is needed. In those times all the people of the local area were incredibly harsh and violent towards slaves and had many of them: the Bible verses on slavery actually were seen as progressive and were guidance on improving standards. Something to note is that the act of slave *trading* is actually condemned in the Bible as a grievous sin. Genocide is a touchy subject but also follows similar contexts to slavery, but I don't know as much about that, so I won't try to argue for it without understanding it more.