Perhaps they are right. Maybe Jesus abolished the entire Old Testament. But this possibility seems highly controversial even among Christians. After all, many seem quite eager to claim that at least some of Leviticus is still relevant today.
Even if we decided to ignore the entire Old Testament and focus only on Jesus, we find some serious problems with the sort of morality he allegedly promoted. In her brilliant essay, "If God is Dead, Is Everything Permitted?" (which can be found in in The Portable Atheist: Essential Readings for the Nonbeliever), Elizabeth Anderson wrote:
Jesus tells us his mission is to make family members hate one another, so that they shall love him more than their kin (Matt. 10:35-37). He promises salvation to those who abandon their wives and children for him (Matt. 19:29, Mark 10:29-30, Luke 18-29-30). Disciples must hate their parents, siblings, wives, and children (Luke 14:26). The rod is not enough for children who curse their parents; they must be killed (Matt. 15:4-7, Mark 7:9-10, following Lev. 20:9). These are Jesus's "family values."This Jesus-figure does not sound all that different from the Old Testament god, does he? Undoubtedly, one can find some positive moral messages in the Christian bible. However, one must be willing to ignore quite a bit to argue that Jesus was any sort of moral exemplar.
This is important because it undercuts much of what Christians - even liberal Christians - want us to believe about the Jesus figure. Even if we are willing to set aside concerns about whether any such person ever lived, we end up finding someone who was far from perfect, godly, or even a model of sound moral teaching.