The article is short. It names all the members of the family. Based on the names, there appear to be:
One adult male
One adult female
Two teen females (yeah, I'm calling 12 a teen)
One male child
There is no information in the article about which one of the above was the shooter, and all of them are old enough to be able to handle a firearm (although it's less likely that the male child, aged 9, would have been the shooter).
Your comments refer to "him" and "this man," so you must be referring to the adult male. Unless you have some information about this incident that is not stated in the article, you are assuming that you know who the shooter was, where there is no information to support that claim.
It seems that you want to believe that it must have been the man, because you believe that men are intrinsically violent. Is it more statistically likely, based on past history? Sure. But you cannot apply statistics that way to come to a correct conclusion about an individual incident.