I had a client who was very intelligent, well spoken, well behaved, socially integrated, knew the difference between right and wrong, but just didn’t care. He killed people who got in his way. Aside from killing people the way you or I would kill a bug, he was absolutlely sane. If being a multiple murder by definition means that one is insane, then he was as insane as they get. If, however, one can overlook multiple murders, then he was absolutely sane. He knew exactly what he was doing, and he knew that he should not do it and knew why he should not do it, but he simply did not care.
Was he insane? Legally, no. He was aware of what he was doing, and was aware of the ramifications. Medically, however, there was obviously something not working in his mind or he would not have been that way.
I think that to some degree, worrying about whether people are insane or not is a bit like chasing one’s tail, for the definition keeps shifting according to society’s perceptions (e.g. up until recently, gay people were thought to be mentally ill). I prefer to look at less abstract aspects, such as whether or not a person knows what society deems to be right or to be wrong (regardless of what the person believes), and whether or not the person is capable of controlling his or her own actions (regardless of what the person feels). Thus in my books, if a person knows that society prohibits murder, and there is nothing inside the person’s head driving him or her to murder, then even if the person does not believe that murder is a bad thing and feels justified in murdering, I would still consider the person to be sane to the point of having to take responsibility.