The cop made one mistake...he took the guy down with a single shot to the chest, but the gunman could have been wearing armor and could have taken the cop out. The proper procedure where you have a homocidal gunman is to fire multiple bullets to the chest and one to the head in case the gunman is wearing body armor. We need to stop being so upset when the police permanently take out a gunman.
I'm willing to bet that more than one shot was fired by the LEO; one struck. Apparently you've never been in a firefight, and do not realize that 4 out of 5 (or more) of your shots miss. Getting multiple consecutive shots on target when the target is moving AND you are avoiding being shot (presenting as small of a target possible yourself) is nearly impossible.
One shot was apparently enough. If the gunman had body armor, he may have taken more, but having seen guys hit while wearing body armor, he'd be an easier target at that point (the vest may stop the bullet from penetrating you, but it still hurts like crazy, does a fair amount of blunt force trauma, and will stun you for at least a few seconds).
The reality is that - even with all the training you can muster - in a firefight you're lucky to hit more than 20% of the time. And that drops immensely if you're shooting for head shots only. You shoot for the largest area of the target as possible, and if your target is wearing body armor you'll slow them down.
Ever seen the damage on the inside of a IIIA vest after being hit with a 9mm round? No, there's no hole from the bullet, but the golf-ball sized protrusion will get your attention in a hurry, if not crack or break a rib. Body armor will stop you from being perforated by pistols, but it won't keep you from being incapacitated.
I've seen guys knocked out of breath - like they were punched in the solar plexus - when hit in their body armor, and it took them 5+ seconds to recover. And the bruises that showed up a few hours later were some of the nastiest black and blue patches I've seen...