It is entirely reasonable to not vote for Donald Trump. However, some NeverTrumpers, though their decision to not vote for Trump is fine by itself, exhibit inconsistencies in their professed philosophies of voting.
Jonah Goldberg claims that he would vote for Trump if his one vote decided the election but otherwise his conscience prevents him from voting for the small fingered vulgarian. This leads one to ask why Goldberg has ever voted for any Republican presidential candidate. After all, his vote has never decided an election, nor could it, and there are doubtless better options than someone like Bush, McCain, or Romney. In fairness to Goldberg, it would not seem that his conscience would prevent him from voting for those former candidates as it now prevents him from voting for Trump. However, there isn’t much reason to vote for an inferior candidate if the only thing that would compel one’s vote is the knowledge that such a vote would decide the election. Goldberg could have submitted whomever he wanted as a write-in candidate and had no less effect on the outcome of those elections.
Catholics who take a Goldbergian position should be subject to greater scrutiny because past Republican candidates have supported evils that we must oppose. Catholics are inconsistent when they endorse evil-doing Republicans in past elections but then claim that a Catholic cannot vote for Trump simply because he supports grave evils. McCain, and Romney both supported at least some level of embryonic stem-cell research, and conservative Catholic favorite Rick Santorum supported waterboarding. Even Republicans like Paul Ryan who hew closer to a moral course tend to adopt foreign policy positions that are difficult to reconcile with Just War Theory. If one wants to argue that one should never vote for a candidate who endorses any grave evils, then not many options are available.
It is entirely acceptable and may even be best to simply not vote or vote for some obscure third party candidate rather than pick the lesser evil among candidates. However, such a position should be held consistently and not adopted only when the GOP candidate is especially distasteful.
There are certainly good reasons to not vote for Trump. Someone might reasonably think that he is not any better than Hilary. (Though it is hard to see how he is worse). A citizen who truly never votes for anyone who supports grave evils would naturally refuse to support the orange skinned torture enthusiast. A Catholic monarchist who doesn’t want to encourage all this democracy nonsense might never vote for anyone as a general principal. It may be an excellent idea to not vote for Trump, but if you decide to join the NeverTrumpers, be sure that your reasons for doing so are rational and consistent.