That is exactly it. Hindsight. In hindsight I'm sure a lot of the people involved would have made different decisions. That's easy, and it's easy for all of us to criticize what they did, given that we have the benefit of hindsight (and a distinct lack of solid information, so the usual outrage machine has taken hold).

This was a very rare event: in the majority of cases, they're able to get volunteers to take a later flight. In almost every single one of the cases where they aren't, the people involuntarily bumped leave without incident. I doubt anyone on the flight crew foresaw their chain of actions ending in violence, right up until it happened. And I think that lack of foresight is entirely reasonable and understandable.

United (as a company) did a poor job of handling the aftermath, though.