Some folks seem to have an idea that either your brain is healthy and works fully well, or then it is not healthy and doesn't work at all. Of course that's not the way it is. A medical condition can affect some brain functions, and leave others untouched.
If somebody is intelligent in a sense that he can plan and operate rationally, but still has serious problems in feeling empathy and estimating ethics, could the person be called insane? Whatever, I'm not interested in debating the meaning of this word myself.
My opinion: Breivik should be put into an MRI scanner before other more delicate examination. No speculation could replace the act of actually looking into his head and finding out the physical situation of his brains.