I think the major part of the problem here is that you don't understand the purposes for which God gave us the Old Covenant. You are also confusing some of the Law given by God with the man-made laws of that culture. More than once Jesus corrected religious leaders who were following the latter rather than obeying God.
To call the mocking of God's prophet a "petty offense" is to compare your personal standard of morality to that of God's. Your opinion of right and wrong doesn't trump God's. He set the laws that the Israelites were to follow and they knew they would be judged by them.
"Behold, the days come, saith the Lord, that I will make a new covenant with the house of Israel, and with the house of Judah" Jeremiah 31:31. This passage was fulfilled in Hebrews 8:8-13, where the author points out the many "faults" of the Old Testament and contrasts them to a new way with better hopes and promises - the perfect New Testament.
The old covenant revealed the coming Messiah. There are over 300 prophecies concerning His birth, ministry, miracles, suffering, death, resurrection and ascension.
The old covenant revealed sin. Paul wrote, "For by works of the law no human being will be justified in his sight, since through the law comes knowledge of sin" Romans 3:20. In other words, trying to live up to God's standard of righteousness is clearly an impossible task as proven by the record of the old covenant.
There were several types of slavery in that culture. The most common type was indentured servitude for paying off a debt. Nowhere in Scripture did God ever approve of abusing slaves.
I'm not going into the rest because you seem to want to blame Christians for the misdeeds of some who either don't understand Scripture or deliberately misapply it. There is only one standard for measuring Christianity - Jesus. He was a Jewish Rabbi who frequently did the unthinkable for a man in that culture - let alone a Rabbi. He spoke freely to women, even a Samaritan woman, lifting them out of oppression and treating them as anyone else. He rescued a woman caught in adultery who was about to be stoned to death as required by the law. He spent time with tax collectors (considered the worst of thieves) and prostitutes. He healed on the Sabbath - forbidden by the authorities. He often rebuked the authorities, embarrassing them by pointing out their hypocrisies and calling them derisive names in front of crowds.
Jesus said, "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished. Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven." Matthew 5:17-20. Sinless perfection is still required for salvation and the Messiah has always been the only means provided by God, all the way back to Genesis chapter 3. The OT pointed to Him and the NT reveals Him. The OT showed that it is impossible to please God by our works. The NT teaches that salvation can only come through faith in the Messiah's works on our behalf.
Ron Paul - Honorary Founding Father