I disagree, which I guess makes you just as wrong? Where are you seeing all of this dark side of Superman, other than occasional shoddy writing to fit a plot? Not a robot, and may have moments of doubt when in gray areas, but pretty much without fail, he's the 'shining example of all that's good' and all that. Truth, Justice, and the American way, etc.
You can easily see Batman going off the rails into Punisher-like territory, but where do you get that inner quality from Superman that would get unleashed like that? Smallville isn't exactly a good example to use, as the writing there is often piss-poor, and it's tough to even reconcile Clark there with Superman. And even there, Clark is always rigid with his moral compass unless the plot calls for him to do something different...