so, anella, i'm taking the killing out of context? or the slavery?
I answered this earlier (killing) in 2 previous posts....GO READ THEM!!! You may not be understanding it because you don't understand Salvation, and what it actually wipes out or atones for, and it's absence in the Old Testament. We are in the "Grace" period, not the LAW as those in the OLD Testament. Is the Law done away with? No, it is handled differently in the dispensation we are living in (GRACE). God will still judge everyone on the Day of Judgement. Take note: The Bible is the Law we will be judged on, but we can escape it by opting for Salvation (now....Grace). Things changed when Jesus died on the cross. This is the pivotal point.
Slaves were mostly taken from other races of people that were conquered in battle. Sometimes, it was just a lucrative occupation for someone to "hire" themselves out to someone as a slave to keep from starving. Slaves were a way of life back then. Back then, slaves were part of the inheritance that were passed from generation to generation. It was forbidden to make slaves of your own race. Most slaves came from tribes of idol worshippers who were not put to death after a war. There were laws regarding the treatment of slaves. Some Jews would have children by their slaves, and sometimes share in the inheritance and blessing as heirs. In some third world countries, slavery still goes on. We no longer own human beings as property (NT). Again, after Jesus came, a lot of this was changed how people were treated called slaves. Also, plural marriages were discounted by the LORD also. It was the hardness of man's heart a lot of what was implemented in the Old Testament, and had nothing to do with God implementing it.
Today, we do not buy or sell slaves, but we still hire other people to do other jobs for us. Childcare, cleaning, cooking, etc. What is the difference? We don't OWN them. There are Filipinos and other races that hire themselves out all over the world as domestic workers. They live with the family and perform "services" for the families that hired them. They get paid a wage, but they are actually hired workers.
You have to take both the Old and New Testaments, and remember that the Old was "types and shadows", and fulfilling in the NEW. While our living situations have changed, the Law has not. It just has been dealt with differently than in the Old Testament when there was no Judgement but God ON EARTH. The killing of offenders was Judged by GOD. Even in the Old Testament, if someone killed someone randomly, it was displeasing to God, and judged accordingly.
That's about as far as I can go into this. I could write reams and reams about why and wherefore, but this will give you a good idea.