Christians wonder what there is in an atheistic worldview that prevents us from doing whatever we please if we think we can get away with it. That question illustrates the fundamental issue with Christian ethics. It is the fact that for them, ethics are not the product of their own intellect or any naturally evolved sense of altruism - they do not come from within - they derive from and are imposed by an external source. Thus, Christians are not responsible for their own morality, or to decide for themselves what is right in a particular situation. Right and wrong are dictated to them, or "revealed", as they call it. They are simply required to obey.
It is ironic that Christians scoff at atheists' supposed lack of moral responsibility, due to determinism. They tend to think that determinism implies that people are nothing more than billiard balls, with no capability to play any role in their own behavioral outcomes, which reveals a laughably naive understanding of human nature. In the Christian view, morality is dictated by God, and it is their "responsibility" only to save themselves from eternal damnation by obeying without question. So which of these two worldviews really embraces personal responsibility, and which sounds like something more akin to a billiard ball? Under atheistic materialism, people act in accordance with their own inner nature and decide for themselves what is right. Under Christianity, God tells them what is right, and the good Christian is simply "following orders" under the threat of damnation.
Christian ethics are essentially selfish. The Christian seeks to derive personal benefit by a combination of avoiding punishment and achieving the reward of a permanent place in paradise. Materialistic ethics, on the other hand, tend to be more altruistic. They do the right thing, not in the hopes of achieving a reward, but because they believe that it is for the best, even if they don't personally benefit.
Christians have a hard time understanding why an atheist should have any sense of morality at all if they don't have to answer to any higher authority. Why not just do whatever you can get away with, if it makes you feel good? It comes down to understanding what motivations we have. They may not be able to believe that people are truly altruistic, because their own perspective is one of selfish motivation. They don't know what it means to be responsible for one's own behavior, because they have always been motivated by the external authority of their God.
One peculiarity of Christian morality is that some Christians believe they have an "escape clause". They can get away with bad behavior without retribution from God, because the only requirement for gaining entry to paradise is to accept and believe in Jesus. This appears to be the case with Robert Dear, the evangelical Christian terrorist who struck in Colorado Springs last week. His ex-wife revealed in an interview that his belief is that his sins will all be excused by virtue of his acceptance of Jesus. And he's certainly not the only one who shares this bizarre sense of Christian morality. This moral "escape clause" is directly attributable to Christians' lack of responsibility for their own behavior.
Christianity also promotes a sense of tribalism among its adherents. Like many other religions, it places people at odds with others who don't share the faith, or who are thought to depart from its tenets. It encourages hatred and division, which manifests itself as bigotry directed at "outsiders" such as Jews, or at "moral deviants" such as homosexuals. This tribalism often results in hostility, violence, or even genocide, with religious faith as its justification.
Another consequence of the Christian worldview is a lack of concern that some of them have for the earth and the people they leave behind. Many of them focus their concerns on their own entry into paradise, to the exclusion of all worldly matters. Some of them are so confident in the coming of the "rapture" that they are happy to see the earth be depleted of resources and ruined, so that future generations will not be left with a livable home. Many even think they will have cause to take pleasure from God's retribution upon the wicked, who have not been redeemed.
In order that the happiness of the saints may be more delightful to them and that they may render more copious thanks to God for it, they are allowed to see perfectly the sufferings of the damned ... So that they may be urged the more to praise God ... The saints in heaven know distinctly all that happens ... to the damned - Thomas AquinasOf course, there are people who behave badly, among Christians as well as atheists. A rational and humane way of treating them would be to try to diagnose and resolve the issues that cause their bad behavior. The Christian way of dealing with them is to seek retribution - to make them suffer for their behavior, and rejoice in that suffering.
This is the true character of Christian morality. The next time a Christian claims that his morals are superior to those of an atheist, be aware that for the Christian, the only thing that really matters to him is his own salvation, and he'd take great delight in seeing you be tortured for all eternity.