We do not know the evidence involved. THAT will determine what happened and how, and who warrants arrest.
Most departments don't want civilians to get too involved in property crimes for a few reasons.
An officer arrving onto the scene has no idea who is fish or fowl, and the wrong one could get shot.
An officer arriving onto the scene could be in danger of being shot.
Now, if the evidence shows the assault on the Marine justified the shooting, it will come out that way. If there was evidence shows something else, then the Marine is in deep doo-doo, regardless of his service. And his friends statements about bashing in windows and only defending himself means absolutely zilch if he wasn't in the vehicle at the time.
Valencia County is not the largest in the state, but it has a lot of rural areas to patrol. That plus a holiday weekend limits deputy coverage.