Do kick that man when he’s down. Essentially, take advantage of the moment when your enemy is at his weakest and most vulnerable.
This is why crime skyrockets during a riot—police authority is spread far and thin, leaving businesses at the mercy of a raging crowd and helpless when the smash-’n-grabbers come to rob.
Be on the lookout for opportunists. The only treatment for this woe is prevention. You cannot foresee every tragedy or bad moment, but try and protect yourself against the ones you can.