Costco Shooting

My question is: Why did the Cop feel the need to shoot the parents too?

https://heavy.com/news/2019/06/kenneth-french/

Only cops are professional enough to carry firearms.

Maybe they were exercising their constitutional right to carry a firearm and the cop thought they might shoot him?

Maybe not. No guns. Aging parents. When was the last time a special needs guy was shot and killd and his aging parents shot at Costco?