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.
I think usually they say something like "I'd put it back but I'm late for my next appointment" or "I already buckled my kid in the car and I don't want to leave him," maybe some variation on "well they pay someone to come gather these up so I don't have to."
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.
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.