Scary stuff. He pulled the trigger once and then waited for the bullet to take effect? Was there something that kept him from shooting the guy a few more times before he closed the distance? Was he shooting FMJ range ammo?
I wouldn't rely on a single .40 cal shot stopping a bad guy. Even if you get him smack dab in the center of the chest he might not stop right away. Even a .44 magnum might not stop somebody if they're crazy enough.