[Politically Correct]

He was shot with a rifle. Unless he was wearing plates (which we have not been told he was), the citizen could have shot him anywhere. Kevlar and other soft body armor can’t stop rifle cartridges.

If you only own a pistol, buy a rifle and train with it.

Worth repeating.
I'll add if you only own a rifle buy a pistol and train with it. The pistol's job is to let you fight your way to your rifle.

[Mr Rogers]

Don’t plan on ever going to church, keeping a rifle in my hands during the sermon. People carry pistols because rifles are tough to stuff in a waistband.

If the guy has head to toe high quality body armor, tackling him will do more than shooting him - and he probably won’t move so good in all that armor. If he has a vest, try for the legs or head. As body armor becomes more common, the typical advice to aim for center of mass might need to be changed.

And from what I’ve read, this guy opened fire through the church walls for starters. Not certain of that, but some reports say so.