Then there are those individuals who are sane, in that their motives and logic are coherent, but who are decoupled somehow from normal human psychology. You can classify these individuals as sociopaths or psychopaths. They are self-aware and in touch with the outside world so that they can muster up a great deal of rationality. But they lack many of the emotional qualities of a normal human. Anders Breiwik may fall into this category.
Finally, there are the larger class of terrorists who I would classify as sane and mentally normal. These are the people who are motivated by ideology or identity toward a stance of dehumanization of the civilian targets. I recall that many of the 9/11 bombers supposedly referred to the people who were going to die with them as “animals” in their personal correspondence. Many of the individuals involved in Leftist and nationalist terrorism in the 1960s and 1970s went on to lead relatively normal lives if they survived and transitioned back to the civilian world. Their actions were justified in broader political or ethical context. “The ends justified the means.” This does not necessarily change the outcomes for their victims, but the difference between these individuals and you or I is more quantitative, of degree, than qualitative. The Irish Republican Army were terrorists, but many Irish Americans could sympathize with their ultimate aims and the rationale for their actions, even if they did not agree with their methods.
I believe conflating and confusing these different varieties of terrorist actors causes problems. Of course the division is not as stark as I present them, these are simply three archetypes which occupy the margins of the character space.