I recall the case and the video but have not been following it. It was reported, in what I saw, that Arbery had left his house in the morning to go jogging. He ran by the two pick-up trucks of the father son team that followed and murdered him and the truck that had the guy with the camera that filmed the murder...who they believe, because he is friends with the father and son, was part of the team and was actually participating in the murder. 

Both vehicles began to follow Arbery after he ran by them. The vehicle with the father and son, both armed, catches up with Arbery as the second vehicle with camera that is filming hangs back. The son gets out of the truck with a shotgun, and begins yelling at Arbery, who eventually turns and goes at the guy with the shotgun in what seems to be a struggle to take the gun away from him. The father who was standing in the back of the pick-up draws his handgun and then the son's shotgun goes off, I think three times. I don't think the father (a retired cop?) ever fires a shot. 

It was the attorney of the guy that was doing the filming that released the video, I think. He claims to show total transparency. 

So, based on what is in the video and the initial stories of the father and son that killed him, it does really look like they just planned to kill him and had their buddy there to film it for some reason. Arbery has no weapon, seems to be just out for a run, and they follow and approach him with guns. 

My thought when I heard it was the prosecutors were just covering for a retired cop and his son.