This case is about when your actions waive the privilege of self defense, when you lose what the law calls "innocence" as one of the elements that prove self-defense.
It is well within the scope of a jury to conclude that threatening to blow a guy's head off and then doing exactly that constitutes waiving the privilege.
Did he threaten to blow his head off?
Did all 3 defendants shoot Arbery? Did all 3 say they were going to kill him? There seems to be some missing information.