Barrett was present to make sure that Britton would be able to kill some kids within the next few minutes.
It's like saying that the hitman's associate who is standing at the scene to make sure that no one is able to stop the hitman from killing his victim is not an immediate threat to the victim's life.
That's not really true.
It's like saying that the hitman's associate who is standing at the scene to make sure that no one is able to stop the hitman from killing his victim is not an immediate threat to the victim's life.
The Barretts drove Dr. Britton from the airport to the clinic. They were unarmed and of retirement age - I don't see that they were very much (actually they turned out not to be any) protection for John Britton.
John Britton could have gotten from the airport using a different mode of transportation, but the Barretts were not "immediately threatening" anyone's life - your definition of justifiable homicide, remember?
According to Hill's statements, he chose to shoot Barrett first. He could have shot only Britton and accomplished his stated purpose for the day. Why did he also shoot the Barretts? They were no threat to babies.