Certainly a common expression, but I got the impression that the posters were only disputing that any of us could be a mass murderer.
There, however, I think Fisty's right. Whether it's Fistful's God, one's own commitment to an internal rule set, or some other external factor the veneer of what we consider "civilized" is remarkably thin and brittle.
Time and Time again throughout the world, history has shown it doesn't take all that much to get most of us to cry havoc and loose the dogs. We are pretty much all not that far from violence.
That's why men like MLK and Ghandi are revered so much. We all know, deep down, that we probably wouldn't have stayed peaceful.