It seems to me that the cop had a hunch that Martinez was a potential scumbag, and that he did pretty well on his approach to not get shot. From what I can gather, there was a foot chase leading up to the shots fired. The gun that Martinez fired at the cop jammed after two rounds, and was reported stolen in VA.
There is much focus in the article on the gun, as if the thing itself was the source of the violence, and not Martinez. The gun is reported like it possesses a margin of extreme violence due to it's menacing looks, its name - MAC-11 - and the fact that it had a magazine capacity of 30 rounds. It has been identified as semi-auto, not full auto, and thus is not a "machine pistol," but that is what the press is calling it because it sounds intimidating and makes the story much more interesting to some.
There is also much ado about a business card from a VA gun shop that was found on Martinez with a quote from a movie; something about cops and paddy wagons.
Overall I think that the cop accurately relied on his training and gut instincts, and the outcome of that was that he survived the shooting of a scumbag. From the standpoint of Martinez, and scumbags in general, I have to wonder why some of them are fascinated with carrying such a stupid weapon. I think MAC-11s are cool, I wouldn't mind having a real one for my collection, and they would probably work great for a home defense role or personal security detail if it were full auto and had a stock, but as for a weapon to be carried they are inferior to a standard handgun which has important things like ergonomics, light weight, and more reliability. For scumbags to be carrying MAC-11s, Uzis, TEC-9s and the like, semi-auto or not, shows that the current breed of scumbag is not as weapon savvy as scumbags of old.