Is the appeal to consequences a fallacy?

For many Christian apologists, a stock argument in their gallery of arguments is the claim that consistent atheism commits the atheist to moral and/or existential nihilism. I myself deploy that argument. Some atheists respond by claiming that's a fallacious appeal to consequences. 

1. Sometimes this involves the angry accusation that Christians are misdefining atheism. Atheism is not a philosophy or worldview but simply disbelief in a god or gods–so we are told. 

i) There's no official definition of atheism. There are multiple definitions of atheism:

https://plato.stanford.edu/entries/atheism-agnosticism/#DefiAthe

ii) In popular usage, atheism is often a synonym for naturalism. And usage drives the meaning of words.

iii) But even in philosophical usage, atheism can be equivalent to naturalism, viz. "naturalism lies at the core of atheism," J. Baggini, Atheism: A Very Short Introduction (Oxford 2003), 5. 

iv) I'd add that the word "entail" has a popular definition as well as a philosophical definition. You need to distinguish between ordinary usage and technical usage. Both are legitimate in their respective domains.  

2. There are several sites on the Internet that list and define fallacies. But I can't help noticing that most of them seem to be run by atheists. These sites exist for the specific purpose of attacking Christianity. That's often obvious from the examples they use to illustrate a fallacy (or alleged fallacy).

Another question is how many of these sites are run by professional logicians, or even professional philosophers. Why would I rely on someone who's not a professional logician, or even a professional philosopher, for definitions of a logical or informal fallacy? 

Moreover, I can't help noticing that these sites define fallacies with Christian theism as the target. The obvious danger that presents is a tendentious definition custom-made to single out the opposing position. That's a made-up fallacy masquerading under an impressive-sounding label. A philosophically serious category of logical or informal fallacies doesn't begin with what you oppose, then invent a corresponding fallacy to invalidate the opposing position. 

If you Google "appeal to consequences," and peruse the sites where that's discussed, there's a lack of uniformity as well as a conspicuously amateurish quality to the analysis. It would be more impressive to quote from up-to-date academic textbooks on logical or informal fallacies. Or articles in journals by professional logicians. 

3. It also depends on what precisely the Christian apologist is claiming. Is he claiming that the consequences of atheism disprove atheism? 

Even at that level, there's nothing necessarily fallacious about contending that certain kinds of consequences falsify a position. For instance, it's often said that global skepticism is self-refuting. Related examples include alethic nihilism or epistemic nihilism. These can be formulated in terms of per impossibile counterfactuals. There's nothing fallacious about that kind of argument. 

4. However, even if we grant for discussion purposes that the appeal to consequences is fallacious when employed to show that something is false, it doesn't follow that it's fallacious to take consequences into consideration when we evaluate the merits of a position. After all, atheists routinely appeal to consequences as part of their standard attack on Christianity. They gleefully quote "offensive" passages from the Bible. They rail against Christian ethics. They rail about how Christianity forces believers to commit intellectual suicide. They complain about how Christianity is a war with science. And so on and so forth. 

5. The fact that many atheists are so defensive about the claim that atheism entails moral and/or existential nihilism demonstrates that they do think that's damaging, if true. Otherwise, they'd shrug it of by saying, "What's the big deal?"

6. As I've documented in detail, many atheist thinkers are moral and/or existential nihilists. In my reading, moral nihilism is more common. Some atheists edge right up to existential nihilism, but blink. Is that because their position doesn't commit them to existential nihilism–or because it's too unbearable to go there, so they slam on the brakes artificially short of that outcome?