You can fool the sum of all people, some or all of the time

Mesmerism involved different techniques, including the placement or brandishment of literal magnets around the subjects, and the monotonous repetition of words and tones, which induced a trance-like state in its subjects.

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In a hypnotic trance, the subject is prone to suggestibility. They tend to believe what they are told and their senses will malfunction to back up these suggestions. Mesmer primarily used the technique to cure various stress-related illnesses but it soon became clear that hypnotism could also be used to make people do things they wouldn't normally do.

Today, any respectable hypnotist will assure you that a person under hypnosis can't be induced to do anything they wouldn't normally be able to do. But then, it's not the respectable hypnotists that you have to worry about. Regardless of their protestations of harmlessness, the suggestibility of a hypnotized subject offers ample opportunity for the hypnotist to wreak havoc.

Aside from the possibility of just ordering the subject to become a killing machine, which is not a reliable technique, one can plant suggestions that allow the subject to justify all manner of wrongdoing (i.e., "Jim is planning to kill you. He will kill you unless you kill him first. You had better kill him in self-defense.").

Hypnotic techniques can also be used to plant "post-hypnotic" suggestions, in which a certain set of circumstances (such as the utterance of a "trigger phrase") cause the subject to act out a preprogrammed behavior. This is more popular as a Hollywood device than effective in the real world, but it can be done.

The main problem with hypnosis as a mind-control technique is that it's pretty difficult to hypnotize someone against their will. That's why insidious megalomaniacs returned to the techniques used by the first Assassins � drugs � while inventing new and exciting ways to manipulate the masses in an economical fashion.