Someone had suggested that there had been two shooters. Not difficult considering that there were three adults. The " clinical " way in which the twins were shot against the frenzied way in which both parents had been killed. So although all were murdered,strange to say that both children hadn't been disturbed for fear of one disturbing the other,so both shot in succession but there was a certain amount of care taken ( for want of a better phrase under the circumstances ) than that of the parents.
A mass murderer/psycho would have performed something that resembled a contract killing without a thought for children let alone adults.
I think the children's toys were close by too,so who would bother in that respect ? A violent killer ?? No !!