Why are murderers oftentimes the nice quiet type (re: Annie Le)?

Because a lot of times these people try and live decent lives and then they either put up with a lot of bullying in their lives and depression or they experience some traumatic event (ie they walk in on their wife having with their best friend) and they just "snap" and commit second degree murder. Usually the ones who commit first degree murder, ie plan out their crimes, are not the quiet types. Except in the case of serial killers. Serial killers are often times the quite types, because if they are organized serial killers they will do everything they can to cover up their crimes, which usually includes keeping up experiences and living a seemingly normal life to the outside world (ie John Wayne Gacy, who was a husband and father and owned a construction business and a lot of people liked him, except he liked to and kill young men and bury them in the crawlspace underneath his house...), but like the first answerer said, appearances can be deceiving.