I disagree with your tactical assessment.
The murderer was not a trained combat veteran, a competitive shooter, or a hunter.
He had plenty to distract him and give incoming opposition an opening.
Taking a .223 round from him was far, far from certain.
High risk, sure. Maybe in the 10-30% range.
The risk of additional deaths of innocents while Scott Peterson waffled and waited...100%.
This is not a suicide standard.
Peterson had no reason to believe there was more than one shooter, and plenty of reason to believe there was only one.
It is clear that Peterson's training on active shooters was sub-standard or non-existent. People do what they are trained to do.
“The murderer was not a trained combat veteran, a competitive shooter, or a hunter.”
And he had no way of knowing that.
The only thing he knew for sure is that the perp had a semiautomatic rifle with deep magazines. And he was shooting kids.