Whatever it was, he certainly wasn’t thinking clearly.
It doesn’t take a rocket scientist to understand attacking an armed man, could result in your death.
You're jogging along, there is a group of 3 white men in a pickup truck down south in Georgia following the jogger, yelling at him to "STOP" claiming they want to talk to him. At this point I'm sure Abery surmised that whatever they wanted or wanted to talk about was not good so he kept going, at one point it is reported, turning around possibly in an attempt to evade or escape them.
When they finally get ahead of him and block his pathway with the truck he goes around the truck only to discover at least one of the men who have been attempting to flag him down and get him to stop, standing there with a shotgun in his hands.
At this point he probably considered himself already under attack and may have reacted by what he considered as a fight for his life.
So can you not put yourself in his shoes just for a moment and think about what he must have experienced as these males were pursuing him? What if it were you and a group of three black guys were chasing you in a pick-up, yelling at you to stop running, that they want to talk to you, but when they exit the vehicle one or more of them are carrying long guns.
You know they train women to put up a fight if necessary rather than to let someone abduct them or go willingly. Maybe they had indicated to him that they were going to do something to him that he didn't want and figured it was better to die fighting than to let them do whatever it was they had in mind for him.
It's not like black men in the south haven't been dragged behind pickups, lynched, burned or murdered before.
And many times, the perpetrators were never prosecuted.