It sounds more like the lab didn't know his own strength as opposed to attacking to kill. BTW, aren't pit bulls the ones whose jaws can lock so they can't/don't/won't (whatever) let go? I understand that any dog is capable of anything but if the story is accurate, it doesn't sound like maliciousness, just overzealousness.
No a Pit cannot lock its jaw; that is a myth.
No that's myth... They're just strong dogs...