Why is it that mass murderers' are usually intelligent and quiet?

[Mr. Snoid]

That's complete bull$hit. There are numerous examples of psychopathic killers who were known as social, outgoing people and charming, smooth talkers.
Being afraid of quiet people is just as ridiculous as being afraid of black people. It's a form of prejudice.
There is no single characteristic that all mass killers have in common. It's a really nasty thing to say, that "quiet people are usually the crazy ones". People who say such things have the mentality of high school bullies.