Two elements stand out to me, as described in the news article:
* The guy who was killed did initiate the violence, BUT except for rare circumstances (e.g. a frail elderly person being shoved) shoving a guy to the ground is not deadly force; Drejka could, legally, have responded with proportional force, but not deadly force;
* Security video showed the guy who was killed stepped back when Drejka drew his gun; at that point Drejka could not, legally, have used ANY physical force.
This was a correct verdict, IMO. This case was unlike the Zimmerman case in that when Zimmerman shot Trayvon Martin, Martin was beating Zimmerman's head into the concrete sidewalk, which was deadly force.