A psychopath is not clinically insane. He is quite rational, often more rational and lucid than most people. He is able to stay calmly focused on his goals over a long period of time. His behavior is not erratic or capricious.Read it all here.
His reasoning and methodology are not impaired, but that does not mean he is not mentally ill. What makes the psychopath abnormal are his goals, which are to varying degrees violent, destructive, depraved, monstrous, and inhuman. His focused lucidity is directed towards deeds that lie beyond the pale of our common humanity.
The psychopath’s world contains nothing but himself, and all his actions are informed by a grandiosity that culminates in massive, murderous, bloodthirsty deeds that will glorify him forever in history, as presented on the stage of his own mind.
Funny, while reading this, I noticed that the definition also closely describes the founder of Islam, the false prophet known as Muhammad. Breivik hates Islam (and for good reason), but he has embraced what he hates: a violent ideology born of megalomania and delusions of grandeur.