I would have to say it is the flamethrower. With this weapon it is up close and personal. Burning its victims to death is a hideious way to die.
Gas is insidious but can be dealt with. Willie Peter is noxious but does not incinerate its victims. Really, it is no worse than large scale HE weapons in its action.
Napalm is impersonal. It is sort of in the same category as nuclear weapons only on a smaller scale.
What really makes a weapon feared and reviled is its effect up close and personal. This is why so many fear the bayonet, which by medical records is virtually worthless as a casuality agent. It is the threat in this case of personal violence that casues it to be effective. The flamethrower is the same way only it is effective.

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Truth is stranger than bullshit!