US liberals and leftists who won’t vote for Clinton, even though that decision will help Trump, who is even worse, are an interesting case study. It reveals a deep difference, not between liberals and conservatives, but between “principlists” and “consequentialists”.
Consequentialists do what they have to do to get the best available outcome, even if the means – and the end – fall short of their ideal.
Principlists feel an inherent wrongness in doing anything against principle, even if the result is an outcome even further from their ideal.
They’re two totally opposed mindsets – ways of thinking about how to choose action – with little scope for persuasion between them.