That's how democracy works though, doesn't it? If you like to think with your head it's unlikely you are going to find another one that thinks exactly like you, let alone an entire party, so you are going to have to vote for the least bad of the lot, pretty much always, I'd say. People are fallible and ideas get corrupted, one of the many reasons why the GOP are successful is that they are of course scrupleless and pragmatists, they have a line, they follow it, whereas the left is often inclined to shoot themselves in the foot. It happened 4 years ago, when they decided that it was going to be either Bernie or dead and from what I read it's going to be the same again.
You are so right about the left wing shooting themselves in the foot. I consider myself on the left. Always have been. Jeremy Corbyn best represented my view and I agreed with the majority of his 2017 manifesto ( though one or two things I had issues with on the last one) . HOWEVER I never wanted him as the leader because I knew he would never win. I had so many arguments with with left wing friends, ( I did help with election campaign '17 ) but I have sadly been prooved right. All we have at the end of it are three lost campaigns and a dreadful right wing govt. As an opposition leader , nothing was won , nothing was moderated. How people still want him and his clones back at top is beyond me. Tories here know how to win. I am closer , not close , to Bernie Sanders than Biden but if I was a democrat I would never have voted for him because if Sanders had gotten the nomination we would all be facing another 4 years of Trump for sure. So I'm still delighted that Biden has won .He seems decent, says reassuring things which is important. Most of all however he seems sane. He has ousted the insane Trump which clearly has not been an easy thing to do.