Non sequitur. You lead with an incorrect presumption.
Witnesses were there, they testified. A sufficient number of witness testimonies were consistent enough to allow the grand jury to ascertain what occurred leading to the conclusion that the officer acted lawfully.
Hardly a non sequitur. Again, who says that Brown tried to "murder" Wilson independent of Wilson's testimony?
As to the lawful part, I suggest, again, that you review the SCOTUS ruling on this.