Likewise, I see the thin line we're walking, and understand and respect what you're saying even though I disagree.
Yeah, Supes and cops are on another level...just trying to make a real-world analogy. Didn't exactly work too well.
Still, no one would argue that someone like Superman is "bad." Even Max Lord would concede the point. But the 'potential' to do evil is what caused him to try to kill them all. Sounds like Shakespeare's Brutus and Julius Caesar to me. Brutus kills him for the monster Caesar MIGHT become. Was he right? I'd say no...maybe you'd disagree.
Most mainstream super heroes try to rehabilitate their foes (thus Batman's villians always returning) rather than kill them. A few kill their foes so that they won't be a possible threat anymore (Punisher on occasion, for example, or The Authority from Wildstorm comics). That's a bit harder line on things, but is still generally a "let's do good" mentality.
Max Lord, on the other hand, is killing good and evil alike (or at least that's his goal). Doesn't Superman have some rights to life as well? Or any of the other heros? To kill the "bad guys" for the greater good is one thing (though I'd still disagree with that approach) but it's another to kill without any sort of moral agenda. Max Lord's nigh-on-persecution-complex for any super powered being bordered on insanity.
Philosophy aside, in simply a contextual comic setting, I'm not following the Crisis issue by issue (though I read most of the lead-ins to it). But there's bigger forces than Earth's super heroes out there (like Anti-Monitor in the original Crisis). Even with many innocents suffering, who is going to beat down the bad forces if not the super heroes?? Evil will arise in any setting, and in comics it generally takes on an epic and insanely powerful form. If all the super powered beings were killed (both good and bad) who would save the world all the time from aliens, interdimensional travelers, meteors, abstract and cosmic beings, etc.??
No, DC needs Superman and his kind. I won't struggle to find a real-world analogy, because there probably isn't one that captures it well enough. But I genuinely think that the killing, regardless of the reasoning, is wrong.