Wait, what? You think Christianity has those things in its “original and purist form?” Is that correct? If so, and I’m not misreading what you’ve said, you haven’t actually read the source material all that thoroughly. Rather, you’re buying into a particular set of propaganda that, as Helio said, cherry picks parts of it to make it look better than it actually was.
To take your first example: ideas about slavery. The bible’s so all in favor of slavery it doesn’t even acknowledge the idea that there are arguments for or against slavery. When it mentions slavery, it’s all about regulating the rules of an institution that it assumes exists and is widespread and will always exist. “People will always own other people” is the underlying assumption.
You can, for example, beat your slaves, provided they don’t die, lose any teeth, or lose an eye. As long as they’re recovered within a day or two, there’s no penalty to the slave owner.
You’re not supposed to keep Jewish slaves forever; you’re supposed to let them go after 6 years of slavery. Foreign slaves for life, however, are explicitly permitted.
It’s OK to rape female slaves, provided they aren’t engaged to be married. If they’re engaged, you have to make up for it by sacrificing a ram.
Most of the religious legalese is in the Old Testament, but the New Testament doesn’t change that. Slaves are instructed to obey their owners as if they were Christ. If their owners are believers, the slaves are supposed to work with enthusiasm for their owners, since being of the same religion is the important thing.