The angry rejection of the idea that one ought to vote for someone she finds objectionable is not only understandable, but I think tied to something deeply important. Voters are being told that they ought to vote so as to minimize harm, which sounds like a moral commandment. But these voters also have a conflicting moral belief — that they ought not endorse a candidate they take to be corrupt. They are being put in the position of choosing an external moral principle over an internal one.
One of the things that Green Party supporters say is that you aren’t supposed to vote for the lesser of two evils — after all, the lesser of two evils is still evil. Rather, you’re supposed to vote for the best candidate.
One way to think about the third-party vote is that it is a form of conscientious objection. Such a vote, like abstaining from voting, allows the voter to avoid acting in a way that she thinks is wrong or distasteful. We can understand this person’s vote for a third party as a commitment not to let the badness of the world force her into violating her principles.
The issue being identified here is not a new one. Philosophers have long argued that, while the consequences of one’s actions are morally relevant, they rarely or never amount to a requirement to act in a way inconsistent with one’s firmly held commitments. A British philosopher named Bernard Williams famously argued that if we were forced to abandon our ideals every time the world conspired to make following through with them suboptimal, this would rob us of our integrity. This is a very compelling idea.