An evil person or persons can commit cold blooded murder without batting an eyelash. Not having a shred of morality does not make you crazy. Moral sane people want to believe that everyone who commits heinous crimes is somehow "off", because they don't want to believe that such evil sadistic people exist. Much less that they are all around us.
Do you think that the German soldiers, that marched millions of Jews into showers, lined them up and shot them, and then stuffed them into ovens were "cracked?" What about the the 9-11 suicide bombers. Clearly evil men, but crazy? No.
It's a cop out to say that everyone who commits heinously evil acts is insane.
You are assuming this is a sociopath doing the killing. If it is a muslim fanatic, your analysis is wrong. A muslim's motivation is religous in nature - he is doing it to get his 72 virgin reward in heaven (that big whorehouse in the sky). Thus, to him, it is a moral act, by the will of his satanic god.