Lot's of replies and I have skimmed them quickly so forgive any repeat information.
The man had his 3 year old with him. That is the key, the crux to the entire situation.
Indeed in many ways. Stepping back from the holster/BUG issue et al, absent other facts not being discussed what kind of guy goes off on another guy walking a 3 year old. In addition to helping any claim of lawful drawing (along the lines of "he did what? The other guy had a 3 year old? Yea he deserves what he got") its just offensive to me.