I don't know if there is "one" reason. We might know more as this case unravels.
Does anyone feel that the type/amount of the wounds might indicate the perpetrator's feelings towards the deceased, or it was just a planned execution but how it went was random? The accused is a good shooter, so maybe the amount of projectiles was somewhat indicative?
To me, it feels that the biggest source of fear and hatred was the father. The next in the intensity of feelings was the 13-year-old brother (as a close in age male, potentially a bigger threat? He was likely also in growth spurt and strong? Or was it jealousy? Or perhaps "hanging the murders" on the brother was a special punishment? Could he feel that the brother was "telling on him" to the parents? Or was the brother the only "potential" person to blame it on, besides himself?). There is clear overkill with the brother. What does it mean? It would seem that the mother was the next in the degree of emotions she elicited - he absolutely planned to eliminate her but she was not the obvious source of fear, so he finished her off later? As to the siblings, as a good shooter, he eliminated them quickly. Initially I thought that perhaps the 11-year-old sister, the firstborn girl, enjoyed special attention from both parents, and probably, even the accused? But now I think that the perpetrator merely overrated his shooting skills - everyone is unmoving, so he became sloppy. Or perhaps, for some reason, calling the police ASAP was the bigger goal than finishing the sister?
In any case, he thought that he had an airtight case with his younger brother being the shooter. (From this standpoint, leaving one child wounded wouldn't matter; he didn't know the sister was the perfect witness, and he himself wasn't wounded, so what if one of the kids was just wounded, not dead? His goal was to take out the three - dad, brother, mom - and call the police ASAP to explain why he survived; barricaded somewhere).
What do people think?