From what I can gather, it seems that the shooter was firing quickly, operating the action and pressing the trigger as quickly as possible. As he pushed forward, the aftermarket vertical front grip broke off or fell off, and his hand went forward of the muzzle just as he pressed the trigger. Shot much of his hand off.
Kel-Tec blames the shooter:
I don't concur. We don't know (from this article) that the guy was firing blindly; he may just have been firing fast (but accurately), which is the point of defensive firearms training. But even if he was firing blindly, guns and their accessories don't know if the shooter is aiming or not, and should not break regardless.“No one should be firing the gun without being ready to fire the gun,” he said. “No one should be blindly firing and pumping like that.”
I concur.
Q: If a gun is intended to be used for self-defense (and not just entertainment), how fast should you practice shooting?
A: As fast as you can.
Q: How hard should you cycle the mechanism?
A: As hard as you can.
If a gun or an accessory cannot stand up to hard use, then it should not be used for self-defense.