Except that I that example, as you say - they have no actual choice in the matter because as you say it doesn’t matter who they vote for.

No you can vote for the non-evil, like I do. But I understand that it is a mature decision to vote who you see as the lesser evil with a chance.

That being said, I think we all do bear some small share of the responsibility for the atrocities our country has committed, if only because we benefit from them

No, not at all? If I am one of two plumbers in a town and someone randomly kills the other plumber I profit from that, but I have 0 responsibility for the murder.

the GOP is doing, all the way up to it’s ongoing attempts to subvert the election process and undermine the justice system, is somehow not their responsibility, despite voting for it.

There is some responsibility, but not exactly the same as if you were a perpetrator yourself.

But I think any rational person looking at the two parties from a utilitarian standpoint of ethics can see pretty easily that the evils of the GOP vastly outweighs that of the Democratic party

Most people are not utilitarian, or at least I hope they aren’t.