I have to agree. Regardless of how the cart being abandoned reflects on someones morality, it's still a pretty small issue that only affects a person who is paid to collect them. To harass someone over it seems like such a petty thing to do, if you're that morally repulsed by the behavior, then take the cart back yourself and don't make a huge scene over it. It's definitely a narcissistic way to handle it. Consider that his time could be more virtuously spent hassling people who park in the wheelchair spaces or even asking for donations to a significant social cause, both of which could be done in this same setting.

If the guy approached me and tried to aggressively assert his moral values over something this petty, I would resist returning the cart even more. Not because I feel I'm right, but because I am not going to be led around like an obedient dog by some punk with a cellphone. Unless he looked like he could handle himself (very doubtful) I would probably triple down on the level of aggression I was shown until he turtled or fled. There are a lot worse people in the world than people who leave their carts out, and I would be willing to momentarily become an example of that to help someone gain perspective.