I've watched the original far more than I would have liked, and I said he was aiming the gun the entire time. Having a version that is zoomed in and cleaned up a bit makes it PERFECTLY clear what the motion was.
Otherwise, explain the motion. Knowing that he had a shotgun in his hands at that very moment, what do you think are alternatives to him raising the gun?
I've already explained and even posted a breakdown in the original thread that Travis moves around the front of the truck with Arbery runs around the other side. At that point, Arbery likely did attack him. But at that point, it is purely self defense.
And as far as I'm aware (I'm no lawyer), any death during the commission of a crime is charged as murder in the state of Georgia, and Georgia doesn't distinguish between first-degree and second-degree.