This argument would sound totally reasonable to other Christians.
After all, Christianity is the only possible source of morality, therefore only Christians can be moral, therefore atheists just walk around raping and murdering everything in sight. Makes sense, doesn't it?
So if Christians are the only ones that can possibly be moral and atheists are inherently immoral, we would expect to see this reflected in the evidence. For example, if we compare all the nations to each other, we should notice that nations with more atheists would have more rape and murder, while nations with more devout religious people will have less rape and murder. Unfortunately, when we compare the nations of the world with each other, we find exactly the opposite: religiosity positively correlates with violent crime statistics. The more devout a nation is, the more crime it has, and the more atheists a nation has, the less crime it has.
If we examine the religion of prison inmates, we see more or less the same thing. Most religions (with the exception of four Asian religions) are statistically more likely than atheists to commit violent crimes.
None of this should come as a surprise to anyone. Plato proposed the Euthyphro dilemma thousands of years ago and showed that you can't get morality from an authority. An authority can only demand your obedience, not give you morality.
And lastly, the most bizarre thing about this assumption is that the most commonly accepted interpretation of the Samaritan parable in the New Testament is that Christians should not assume moral superiority.
This assumption is simply delusional on every possible level. The evidence shows that it's not true, philosophy/logic shows that their claimed source of morality cannot possibly provide them with morality of any kind, and their own magic book warns them against the assumption of moral superiority. So why do they do it in contravention of the evidence, logic, and their own magic book?
As Dan Dennett says, religion is a trap baited with people's desire to be good.
As with most religions, the followers are told that their religion is the only possible way they can be good people, or at least that following the religion causes them to become better people. The implication is that if they ever leave the religion, they will suddenly become more immoral, and you don't want to become immoral, do you? If you don't want to turn into a serial killer, you'd better not ever question the teachings of your religion! Nosiree! If you ever question the religion, you might leave the religion, and if you leave the religion, you will instantly become a monster just like the famous serial killer Richard Dawkins, who roasts babies and worships Satan.
*sigh*