Why did the Ferguson prosecutor go out of his way to attack witness credibility?

In his statement announcing that the grand jury had decided not to indict Ferguson police officer Darren Wilson for shooting Michael Brown, St. Louis County Prosecutor Robert McCulloch took the unusual step of attacking the credibility of many of the witnesses in the case.

McCulloch criticized witnesses for being inconsistent in their statements to the media and law enforcement, and for offering accounts of the shooting that were contradicted by the physical evidence collected from the scene:

In any criminal investigation, some witness testimony will turn out to be unreliable, inconsistent, or even false. (Eyewitness testimony is, in fact, notoriously unreliable.) But McCulloch’s decision to attack the credibility of witnesses when announcing the grand jury’s decision raises questions about the way his office approached this case.

If McCulloch believed that this evidence was not credible, then why did he present it to the grand jury? It is perhaps understandable that he would have presented evidence with only minor credibility issues, in order to let the grand jury evaluate it. But McCulloch referenced “witnesses” who had only heard about the shooting from their neighbors, or from the media. It is hard to imagine a reason why it would have been reasonable to present that evidence to the grand jury.

And if McCulloch didn’t present that testimony to the grand jury, then why discuss it during the press conference? What would be the purpose of bringing it up at all? By attacking the credibility of the eyewitnesses to the shooting, most, if not all, of whom had been publicly critical of Wilson, McCulloch gave the impression that he was acting as an advocate for Wilson.

Wilson is entitled to a robust legal defense, but that is a job for his own attorneys — not the prosecutor.