They might agree that they would be a better person if they returned their shopping cart. But how many would sincerely state "I won't return the cart because I don't care about putting any effort into being a good person"?
I'd bet you'd get a litany of complaints about the parking lot, the cart, the store, the weather, anything but their own decisions.
Yeah. What it boils down is "I don't want to put in even a tiny effort or suffer any inconvenience to do a simple chore that helps other people"
Which isn't something I think good people say
sure, but that's my point. People doing objectively bad things all day think they are doing just fine.
It's the inverse of the essentialist fallacy. Faced with a bunch of evidence (if they cared to look) that they are consistently making life worse for themselves and everyone around them, their reaction is "I am not a bad person. I didn't even really do bad things, I just did the same stuff anyone else would do, plus it wasn't that bad, plus I like to think I'm awesome, so that's what I think."
BTW I think this is an inherent human trait and I have it too. Some are just really good at it.