Except that he didn't do the killing to promote his manifesto (I thought that was a bit weird in Anonymous' statement), he did it to harm the Norwegian Labour Party.

Quite successfully, unfortunately. Those kids in the camp, especially the older ones, were probably going to fill the ranks of the real Labour party later on in life, politicians of the future. Now their future intelligentsia is either dead or suffering PTSD. (also remember that Norway is rather thinly populated)

That was his goal. To hurt the Labour party, and he hurt them bad.