Why did he do both these things? I would guess, to cause the police to be busy in Oslo so he could his evil and sinister deed on the island. (But this is just my guess).
Another scenario was that he found out that his main target was not at the location. So the island perhaps was some kind of twisted plan B.
Or he figured how to do as much damage as possible in one day.
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Sorry but there will be a few more posts about Anders Behring Breivik and the norway massacre on the island Utoya.