This reminds me of the trolley problem. One candidate wants to kill five people, the other "only" wants to kill one person. No matter what you do, it is guaranteed that one of them will get elected and kill at least one person - but if you try to use your vote to make the lesser evil slightly more probable - you are suddenly complicit.
Even worse - if the kill-one-person wins and kills that person, the kill-five-people candidate' supporters will be the ones to hold the kill-one-person voters accountable for it. Their candidate would have killed more people, but because he lost the elections he was not able to kill anyone, which somehow makes voting for him more ethical?