I have a list of DS9 episodes to skip if you only want to see the show's many high points. You could just go with any episode with an IMDB score of 8.0 or higher, but I like to dig into these things.
People defend the Punisher, but that's only because he never makes a mistake. I don't think he's ever shot the wrong person (it was a big deal the one time he thought it happened) but that's not an endorsement of his conduct so much as it is an artifact of him being fictional. I mean, I could write a story about a boxer who cures sick people by punching them. He's never failed to cure someone, even if they were dying, but that doesn't make punches an acceptable form of therapy in the real world.
A course of action is only good if it is a good thing if everyone in the world was to follow it. So, imagine an average person. Imagine how they'd do as a deadly vigilante. Granted, they have no training, so let's imagine your average cop or soldier. How well would they do as a one-man justice system? How much collateral damage would they cause?