What a terribly tragic accident.
Sorry, don't buy it. This was no accident. Pure and simple negligence with a firearm.
First, he broke rule #4
1: Assume All Guns Are Always Loaded - Make NO Exceptions
2: Never Point A Gun Where You Are Not Willing To Shoot
3: Keep Your Finger OFF the Trigger (Until You Are Ready To Shoot)
4: Know Your Target And What Is Beyond
Second he broke the rule of common sense using a 9mm pistol to shoot at a snake in a birdhouse! Just stupid.
You are correct about it being tragic, but there was nothing 'accidental' about it.