Some people are sanely insane: do not underestimate these people.
It is quite a talent to want to accomplish something that is ill advised, destructive, and wrong—and to convince others to follow you by appealing to their reason. Nevertheless, some people have this talent. They can make a phlegmatic, cogent argument for engaging in everything from cheating on an exam to perpetrating genocide. They speak calmly when suggesting a course of action, do not seem upset when you challenge them, answer your questions with patience, honor meeting etiquette, and generally comport themselves in a professional manner. They know exactly what needs to happen to achieve their goals, and they pursue such goals methodically. Often, their appearance belies their motives. When a person possesses the above qualities, their potential to do harm to others greatly increases.
Lest one should fall prey to such a person, this aphorism bids one to separate the logic of a message from its delivery or its deliverer. If the logic being espoused is disturbing—no matter how elegant or consistent it might be—consider the damage that might result from the implementation of any actions resulting from such thinking…and take a highly skeptical, concerned, and guarded stance toward the person speaking. If you don’t, they’ll convince you, too.