Caporegime
I bet it was a faulty gun\switch.
It wasn't faulty at all. It worked perfectly, which is why the guy is now dead.
He had let her fire the gun on semi-auto. Then he switched it to full auto. She fired the gun and lost control because it was too powerful for her to handle. The barrel rode upwards and sent several shots into the guy, at least two of which struck his head.
This is what happens when stupid, irresponsible people are given access to firearms.