Ok, this could be a random coincidence but... If not, you have to ask, who benefits? Or maybe who might have something against the witness?
As to who benefits, well that could be either those on the side of the prosecution or the defense. Consider, if the prosecution had coached or made some kind of deal with the witness, they might not want that coming to light. OTOH, the defense might be planning an appeal and not want to face that testimony again in court. Sure, prior testimony would probably be admissible, but would lack the punch of a real person and the defense could also go against it in ways that cannot be refuted. Of course, to buy into either of those theories you have to have a healthy mistrust of a whole lot of people who are supposed to be trustworthy...
As to who might hold a grudge? Could be someone doesn't like the idea of LE going to jail... Could be some completely 3rd party here who was engaged in something else with the deceased witness, who didn't like the idea of him getting too buddy-buddy with police and talking.
Some other aspects of this are interesting. Shot in the mouth and "riddled" with bullets sounds odd. The mouth shot would certainly seem to be sending a message - don't talk or we take away your ability to talk. That could just be coincidence though, a byproduct of being "riddled" with bullets. That facet is much more interesting. That speaks to an amateur rather than "professional" hit.
A pro would use an appropriate caliber and fire once (head) or maybe twice (chest/head). One or two shots tends to be disregarded as possibly fireworks. A whole bunch of shots (ie. "riddled") can't really be mistaken for anything but a lot of gunfire - and that draws attention, something a pro would be trying to avoid. Lots of shots shows emotion, anger, fear, hatred. It also shows a lack of appreciation of just how much (or how little) it takes to kill someone. Of course the flip side is, if you're a pro and you've been instructed to make it look like something other than a pro hit...
Your comment about a 3rd party sounds right on the mark. This murder seems too much of a coincidence to be random. He broke down crying on the stand. Maybe he knew he was putting his life on the line by testifying? That doesn’t necessarily mean that he himself was involved in any shady dealings, but he might’ve known people who were. So sad...
It turns out, this young man was shot last year, too:
https://www.freerepublic.com/focus/f-news/3784256/posts