I think it's funny how absolutely feral some people get, seemingly over a random guy calling them out in jest for leaving a cart stranded. I believe they're so mad because they know they've been caught with their pants down; they've failed to extend a simple courtesy to their fellow shoppers due to their own sloth. If they truly thought not returning a shopping cart wasn't a big deal, they'd treat the Cart Narc like they would any other garden variety vagrant and simply avoid confrontation. But they don't simply brush him off: they treat him like a threat because he threatens their sense of morality; he disrupts the idea they have in their head that they're a decent member of society, and the false idea they wouldn't be a morally-bankrupt savage if it weren't for the threat of punishment.
Cart Narc is waking up the cart-deniers and forcing them to face the truth, and for that, I salute him.