A couple of problems, though. First, the nails are coated with a sort of glue (that's what keeps the strips together). When they go into you, they hurt! (I was framing a building addition years ago, and had the wall laid out on the ground--a knot I didn't see deflected the nail upward in the sill plate and into my hand instead of the stud. First reaction was to pull my hand back (wrong way), until I realized I needed to push it forward to get it off the nail).
Doing that to yourself would require some serious determination (or anaesthesia), and would be an odd choice for a tool of self-destruction.
Second, even back when, these devices had a safety mechanism to keep some idiot from shooting nails all over a job site willy-nilly. You have to push the front of the nail gun against something you want to nail. I'm not sure soft tissue would work well enough unless backed by bone, but that extra effort should have left a bruise or some mark, at least part of the time.
Yes, those can be thwarted by wiring the safety back, but who'd want to, and how many bankers would know that?
Been there, almost done that - except it involved my Wolverine work boot. GEESH it was hard to get my boot loose from that cement coated sinker (framing nail).
Even wiring the safety back, I couldn’t make my butane powered Paslode auto-fire. Ya just can’t get repeated explosions to fire the nails.
Sure, with a compressor-powered framing nailer, but did this guy own one? know how to use it? etc etc.
Just to understand, I’d like to know more about the crime scene and the tools involved.