This is my first post (and the first time I've read the NSMB.com forums) in several years, but hopefully I can contribute something.
Cam, awesome OP, lots of thought provoking ideas and lots of good replies as well.
It's surprising that in all the comments, nobody has made mention of the most notorious instance of herd mentality from the 20th century, that of Nazi coercion of ordinary people to do horrible things to other human beings. Granted, there could be nasty physical consequences if you didn't do what they wanted, but none the less, a pragmatic modern society somehow allowed persecution of citizenry to run rampant, with very little resistance.
Then take the Milgram experiments - on the basis of the fake experimenter's assumed authority, a lay-person can be persuaded to zap a (fake) victim with electric shocks. Not everyone goes to the peak voltage, but the fraction of ordinary common-sense people who do is significant.
Then take the the Standford Prison experiments - fake prisoners and fake guards go bananas on each other with little provokation other than a license to act.
I'm sure we can all think of instances in our lives, as Cam suggested, where we do things that aren't quite angelic in nature but we justify it in many ways. Hopefully these acts don't involve inflicting direct suffering on others, but clearly that's not so unlikely either.
So what? The point is that, if not all of us, then a huge (majority) fraction of our populace can, will and do behave badly, even brutally given the right conditions. An enormous mob of 100,000 and a handful of cops essentially turns the tables on authority and creates a new social norm of breaking windows (snitching on your neighbour to the SS), looting London Drugs (delivering electric shocks) and stealing mannequins (stripping a fake prisoner and leaving them shivering in a cold cell overnight). Am I (or is Cam) saying that this is a legit excuse and that makes it OK? Nope - clearly there is responsibility and will, in particular with the instigators and provocateurs; I'd also suggest that age and experience can cushion us from social pressures and firm up our internal convictions regarding right and wrong (hence the rioters were mostly young, dumb and full of PnV). But we have to acknowledge that ALL humans (yes, even me, even you) have enormous capacity for good and ill under good and ill circumstances. I share the opinion that a hockey riot is hardly a life and death situation that places such duress on individuals. However the sometime ephemeral nature of our moral compasses is very real, which accounts for why that theme is so prevalent our history, literature and philosophy.
Quote time:
"the line separating good and evil passes not through states, nor between classes, nor between political parties either, but right through every human heart, and through all human hearts. This line shifts. Inside us, it oscillates with the years. Even within hearts overwhelmed by evil, one small bridgehead of good is retained; and even in the best of all hearts, there remains a small corner of evil."
Points to who can name the author!