My opinion based on observation and apparent patterns of sociopathic behavior.
Some write their plans out because they are obsessively organized and want records kept of everything.
Some write their plans out because, sadly, they are proud of whatever diabolic plans they have made.
Some expect these plans to be discovered and read to the public, long after they themselves have died in pursuit of their goals. They expect to live on in infamy, or become the latest Celebrity Madman.
The news says that Breivik was his idol. Breivik wrote a 1,516 page book and placed it in the internet to be seen after his deed.