Murder as a charge requires malicious intent for it to stick, as in deliberate intent to kill or maim.

Different guns have different trigger pull weights, some only requiring a slight brush to go off (sometimes called feather-weight). That said, firearms training always emphasizes never flagging anything that isn’t the ground or a target, to always keep one’s finger off the trigger until ready to fire, and to store firearms unloaded(this rule is broken all the time, though). so at minimum this guy was blatantly negligent with a firearm and is a shoe-in for manslaughter.