He pulled the trigger, that legally makes him part of a involuntary homicide!
Involuntary homicide is not a crime. There must be some sort of voluntary action to make it a murder or manslaughter. For instance, if you start your car, not knowing that someone is sleeping under your car, and you kill the person sleeping under the car, you definitely committed a homicide by starting the car, but did not commit a criminal act. That would be an involuntary homicide.
Exact laws differ from state to state, so someone is going to respond it is different in such and such state, and they will be right... BUT, the general frame work is:
Murder is when you intended to kill the victim with full culpability.
Manslaughter is when you did not intend to kill or had some other form of extremely diminished culpability, but you did intend to harm the victim, or do a clear criminal act.
Voluntary manslaughter is when you intended to hurt the victim, but accidentally went too far and killed the victim.
Involuntary manslaughter is when you did not intend to hurt the victim, but led to the death through a criminal act.
Constructive [voluntary or involuntary] manslaughter is when you actually acted.
Criminally negligent [voluntary or involuntary] is when failed to act, and that was the crime.
AND FINALLY:
Involuntary homicide is when you caused a death, like starting your car, but had no criminal intent, like not knowing someone was sleeping under your car.