In a 1500 page manifesto, the terrorist explains his use of Modern Warfare 2:
I just bought Modern Warfare 2, the game. It is probably the best military simulator out there and it’s one of the hottest games this year. I see MW2 more as a part of my training-simulation than anything else.
The killer published the manifesto, in English, giving a view into his violent philosophy and the extensive planning that took place before the attack. Breivik was considered extreme right-wing in his political stance.
Adds Breivik in the manifesto (via ITP):
Once you decide to strike, it is better to kill too many than not enough, or you risk reducing the desired ideological impact of the strike.