Why?
First of all who shoots someone in the head when he stated himself that the boy had the gun to the cashier head.
It really makes sense when you think about it. Even a lethal shot to the body (heart) can take about 25 seconds to stop the perp, more than enough time to kill the gal or return fire, while a shot to the head can instantly stop the threat.
I agree it was a tragedy, but the tragedy was of the bad guys making, Jason Lee's actions were correct.
You DO understand that I was quoting a stupid lib who posted this on the linked webpage.
If it made no sense, then the lib was successful.