He's a white nationalist who opposed immigration. He bombed the Prime Minister's office because the PM is from the (progressive) Labor Party and supports immigration. The bomber/shooter claims to be a member of the Knights Templar and to be fighting against Islamization and mixing of races in Europe. He called this his act of martyrdom and hoped that it would incite a race war.
Here is a book he sent to friends, it's 1500 pages of the ideology behind his beliefs and attacks.
2shared - download 2083+-+A+European+Declaration+of+Independence.docx
If you post anything from it or repost this link anywhere, please do not post the picture of him with his sister and stepmother, and ask others to do the same. They do not share his hateful ideology and do not deserve to be dragged into this; at one point he mentions that he feels it's unfortunate that his stepmother would likely be killed in an attack on Norway's immigration Deaprtment, but that he has to accept that casualty because she is a race traitor for allowing Muslims to immigrate to Norway.