I'm guessing that he was in "extreme fear" that the six year old was going to kick him in the shins, or punch him in the crotch.
IDK. We had a kid that age a few years back lock and block a nursery door, pour gas on their new baby sister, set her on fire, and then refuse to open the door until she was dead. Not saying the guy in the OP is justified. Just saying psychopathic children can be just as dangerous as the adult version.
No, they cannot be 'just as dangerous.' They can certainly kill but they are not as dangerous as they are utterly incapable of overpowering an adult. All three of my children (and one is a teen) could not move me one inch should I decide I don't want to move let alone cause me harm. A child needs to hit you unaware and with a weapon in order to put you in danger and this man was clearly aware.
Of course this does not consider if the child was armed with a gun (still not as dangerous as an adult with a gun) but I would be shocked if that were left out of the story. I hope the media has not slipped that far...
I do wonder what the man was actually doing though, I am not entirely sure I could shoot a 6 year old no matter what the situation.