You're funny, the guy almost killed a cop, that was attempted murder. You see that happen right in front of you and try to remain calm. Sure cops aren't supposed to do that, but they're still human. Sure they deserve to be punished somehow but not fired, anyone in their shoes would have done the same.
Exactly you have someone about run you or a friend over and the adrenaline kicks in you don't think about what your doing its basic survival instinct taking over at that point.