That's the part I don't get, but what we don't know is:
How old were the kids? (I read in their teens somewhere, sorry, no link)
Is this a political summer camp'? (I read somewhere that the island was owned by the same party as the PM).
If these are the children of the political party elites that he rails about being the ones subjecting Norway to Islamic multiculturalism, that may be why they were targeted.
That does not excuse killing children in my book, ever, but I can see where that might fit into some sort of twisted logic. I'm not so sure I want to get that far inside this guy's head.
My theory on why at 143.