If a psychopath says "tell me which of your kids you want me to shoot, or I shoot them all anyway", you don't become an accessory to murder by answering him, because everybody recognizes that you were under duress and just trying to reduce the damage.

If you are faced with a choice of "tell us which jerk gets to order everyone around, or we'll pick someone anyway without your input", then attempting to pick the lesser evil also does not make you culpable of anything, for similar reasons.

(If it was possible to send a "protest" message by not voting, things might be more complicated, but in practice any such protest signal would be undiscernable from apathy and cynicism noise)