What I think is the guy was walking down the street, the cops yelled at him, he turned around and tried to sell them a KitKat bar. Of course, the officer bought a Kit Kat bar thinking "mmm I like Kit Kats". However, as the boy was walking away (with his hard earned new dollar), the officer took a bite only to find his Kit Kat was stale.
Here's where it gets a little confusing. I think the officer leapt out of the car and ran after the boy yelling "Hey I think you accidentally gave me a stale Kit Kat Bar", but the boy heard "Damn you black thug, I'm going to kill you". So the boy turned around with a magical "single use" ray gun from the planet Zoltan (which he had received in trade for a previous Kit Kat bar) and fired at the officer. The officer, having actually been a stand-in for Jackie Chan (little known prior work experience the STLPD has been maliciously witholding from the public), was able to nimbly duck the laser beam shot like Keanu Reaves in that Matrix movie.
At this point, I think the officer was unfortunately NOT aware this was a misunderstanding (perhaps malicious sale of stale Kit Kats in addition to attempted murder), had but one course of action. I think it's prudent to note that it cannot be reasonably expected the officer KNOW this ray gun was of the Zoltan single-use variety from a distance of 30 feetmeters, and would vaporize in seconds. Nope, he probably thought it was a NORMAL ray gun and another shot was likely imminent, and was forced to return fire in self defense.
Unfortunately, when the boy collapsed all of his Kit Kat's scattered around the pavement as the single-use Zoltan ray-gun vaporized. Now, it's a hot summer day and after laying out the police tape I imagine the cops were hungry. That's hard work in those conditions... so I'm guessing they probably ate up the leftover Kit Kats without thinking about their role as crucial evidence in my theory.
There's been pretty much nothing in the 29,201 eyewitness accounts that disproves my theory.