Why haven't Christians learnt anything from their dark and bloody past? Why are they still so judgmental?

Slavery in the Old Testament was a result of the fact that nations conquered other nations and rather than kill the non-soldiers/citizens; they took them and gave them a home. They owned them in a sense, yes, but they also provided for and protected them from others that would prey on them. Being considered a possession was actually better than the fate that they would have had (if they were lucky enough to get into a home with someone kind.) God admonished believers to treat them with kindness and not to be abusive. God did destroy evil, baby sacrificing, idol worshipers who would have tried to annihilate the Israelites. He did this, yes, to insure that the lineage of which the Saviour would be born would survive. He did so to enable mankind to have forgiveness for sin. Today, God offers that salvation to all who would receive it (including gays) and those who have had abortions can be forgiven if they seek the Lord.