I feel shame when I meet them, but they do exist. And the rest of us shouldn't get a pass because we suck.
You haven't seen them at their very worst, in the moments where they did the things that haunt them for decades.
That's the point here - some people do less nasty stuff than others, but all of us have treated others horribly at some point.
"Pieces of shit" may not be the best phrasing. I'd say "When you meet a person, no matter what they seem like, you can be highly confident that they have done something wonderful for someone at some point, and that they have mistreated someone horribly at some point."
And can't you just accept that some people are good?
Everything people do is out of some sort of greed. Some of it results in good deeds others not so much.
You volunteer or donate to charity because it makes you feel good, boosts your ego, and/or inflates your sense of self worth but you ARE being helpful in the grand scheme of things.
If you're consistently doing good for reasons which are in no way indistinguishable for pure goodness, to me it's the same. Call it "boosting your ego" or "inflating your sense of self worth". Some people just feel good knowing they've helped other people. I don't see how these people can be considered "shit".