"Mind your own business" is such a perfect encapsulation of how completely incapable of self-reflection that person must be.

The cart was no longer their business, but yours. So not only couldn't they recognise that the judgment they felt came from within, they projected that feeling outwards so hard they ended up sticking their nose into your business about it.

That's how they avoid learning basic life lessons like, "I should return the cart," because as soon as they hit the "I should" part they freak out and make it everyone else's problem.