That is an excellent point.
I think it along with all the stories like these (remember that SWATing of the Marine a few years back) provides some evidence that the police [at least a non-insignificant portion] think of themselves as the unquestionable authority and want to make it known/felt. In the case you cite, the Citizen even being armed was a threat to his authority (under the same reasoning that gun-confiscators wish to take your guns), and in the marine's case it was so that they could shoot someone (see how they crowd into the doorway?).
I dont think so, watch the video again, the cop was clearly freaked out. He knows the guy has a gun, sees him reaching for something, freaks out and pops the guy. Sad day for everyone.