Ah, but by your own admission, the best possible choice is actually to start a revolution, not to vote for a party that is also evil. So why doesn't your unique take on morality instead assign blamelessness solely to those actively trying to start a revolution, instead of conveniently assigning blamelessness to those that vote for the lesser of two evils but perpetuate a system you admit perpetually ratchets up in evilness? Is it because its easier to tick off something on a piece of paper and tell yourself you're doing your part rather than devoting your life to actually making things better?
Seems to me that if you really want to have this black and white view where only, and I quote, those that make "the best possible righteous choice" can be blameless, then you unfortunately fall into that ... other bucket :/