i) Trump supporters routinely say not voting for Trump is a vote for Hillary. I've seen a lot of NeverTrumpers rankle at that charge. As a NeverTrumper myself, I'm not bothered by the charge.
Given certain assumptions, there's an obvious sense in which the equation is true. It doesn't mean NeverTrumpers are voting for Hillary in the sense of going into the voting booth and pushing the button next to Hillary's name. They're not casting a vote for Hillary in that direct sense. Rather, the point of the claim is that by declining to vote for the only candidate who can defeat Hillary, the side-effect is to help elect Hillary. And up to a point, that's a logical inference.
ii) Mind you, that depends on certain conditions. It presumes that Trump is competitive with Hillary. If he's too far behind, then not voting for Trump doesn't make a difference.
Likewise, if you live in a blue state, then even if you vote for Trump, that will be swamped by Democrat voters.
iii) I think NeverTrumpers are too defensive on this point. It sounds bad to say not voting for Trump is a vote for Hillary because it's taken out of context. Considered in isolation, it sounds culpable to do anything that would contribute to Hillary's election. But there's more to it than that. The problem is that Trump primary voters backed the rest of us into a corner.
To take a comparison, it's normally wrong to embezzle bank funds. But suppose bank robbers kidnap the family of the CEO or CFO of Citicorp, using the hostages as leverage to coerce the CEO or CFO to set up a Cayman account just for them, then transfer $10 billion from the City Corp account into the Cayman account, to secure release of the hostages.
Indeed, you have movies with a dramatic premise like that. Take Harrison Ford's Firewall. (I haven't seen it, just read reviews.)
That puts embezzlement in a different light. The audience is supposed to be sympathetic to the CEO or CFO because the robbers put a gun to his head. That forces him to do things he wouldn't ordinarily do. There are attenuating circumstances that exculpate his actions.
By the same token, NeverTrumpers feel they've been maneuvered into a situation where the acceptable options were taken away from them. It's not their fault that they refuse to choose between the remaining, unacceptable options. The chance for a good outcome was already nixed during the primaries. They aren't responsible for the dire consequences which irresponsible voters set in motion. The results of the NeverTrump position is mitigated by the situation into which they've been thrust, against their will.
It's like someone pulling the pin on a grenade. Once he does that, it may be too late to prevent the ensuing damage. The only way to prevent the damage would be by not pulling the pin in the first place. But if that happens, there's no going back to the options you had before he pulled the pin.