They're not true Christians
"They're not true Christians," is an oft-uttered phrase among Christian apologists used to "disown" examples of self-proclaimed Christians who have committed inarguably immoral acts. Frequently, the same Christians who will use this phrase will also criticize atheism for lack of moral oversight, often even going so far as to point out immoral supposed atheists and try to extend this to atheism as a whole -- a form of flawed inductive reasoning. Furthermore, it may be argued that they are using the fallacy of special pleading, wherein the apologist holds themselves to a different standard than the source of their critique, as "true Christian" would, under their definition, never commit immoral acts. Often, such apologists ignore the large number of atheists with outspoken dedication towards moral principles, either informally or through adherence to schools of thought such as secular humanism. Clearly, such a statement merely serves to whitewash Christianity as a bastion of morality, while juxtaposing the supposed dangers of atheism and the atrocities it may bring.
More pragmatic examination of this argument yields further weaknesses. First, one may ask, where is the line drawn for when one ceases to be a "true Christian?" Clearly, some immoral acts are far worse than others, but when does a self-described Christian cease to become a "true" Christian? Lying, under Biblical definition, is immoral, yet finding a single person who has never told a lie would be quite difficult. Assuming that to be an "forgivable" immoral act, one can invoke a myriad of other acts, from infidelity to theft, that we know, time after time, self-described Christians have perpetrated.
Throwing even these aside, consider the number of death row inmates who claim to have found God and express seemingly legitimate remorse for their actions. Though few would support the release of these prisoners, many evangelical Christians support these conversions as a way for these inmates to find salvation. Are these "true Christians?"
Ultimately, this argument generates an ultimate problem for itself. It's impossible to know who a "true Christian" is before they die. At any point, a supposed Christian could do something "horrible" before then. Examine any particular Christian and try to evaluate whether this person will ever commit a "horrible" act. It can't be done, so there's no way to tell.