BillCA did a good job of predicting the eventual legal outcome of this tragedy. An Involuntary Manslaughter charge seems fair and reasonable to me, at least based on what we see in the video.
I have to say though that this is one of the strangest things I've ever seen. Unholstered his weapon, almost instantly fired, then re-holstered his weapon and threw up his hands as if to say 'did I do that ?'.
I suppose we will never know what was actually going through his mind when he did it, or as far as that goes he may not even know and/or remember. Glenn E. Meyer's suggestion that it was perhaps stress related may be on the mark.