The chance of ever knowing the 'full' David-side story is nil now. That is truly a shame especially for the reason David can't speak for himself.
But it is inescapable that he was in their midst. He had a vehicle and must have 'associated himself' into the situation either by picking up one of the thugs (even in a humanitarian act like giving him a ride), or driving to the location of the initial bludgeoning. Starting with that it's just details.
And I fully agree with the second and third points in your comment. Those who are responsible (and SOMEONE OR MORE was responsible no matter how good their defense attorney's are) need to feel the FULL penalty for what they did. We don't need minds like theirs in our civil midst.