Obviously the unarmed victim had no regard for his safety. People aren't rational or logical. That goes for the shooter too.
People are not expected to act rationally, or logically when a weapon is pointed at them.
"We can't have it both ways."
If we expect people in these situations to act with all logic and rational than we can not say, "train, train, train, because when somebody points a weapon at you, your fear and adrenaline is going to be engaged, and you will lose your ability to act rationally and logically."
Here is just one article of, "tens, of thousands," that will tell you an emergency situation requires significant training to handle effectively.
https://www.gunholstersunlimited.com/blog/why-firearm-training-self-defense-training-is-a-must/
The very second the shooter exited his truck, and picked up a rifle, the dynamics of the situation where such: a man we know who has a rifle, and a victim we know, or assume to be unarmed.
No matter how logical or rational a person is at any given time, in the face of a weapon, and we in the self defense crowd say this all the time, "you revert to your training." Most people, have no training.
Do we know what the victim was saying to man who was pointing a rifle at him? It could have, "you are big piggy, and you should play in the sand box," and it also could have been, "put the gun down man, why do you need a rifle, come on put the gun down."
We don't know the audio. If anything like, "put the gun down," was said, than I have to assume the victim had regard for his own safety. The victim had no training, no idea how to handle the situation.
Frankly, I'm not sure how I would do in a situation like that... Somebody pointing a rifle at me? I'm not sure, I'm hoping I would be fast enough and well trained enough to defend myself.