Nope, that's why I said usually but if you look at most serial killers they are usually the quiet ones. Take for example the Columbine shootings and the Richard "The Iceman" Kuklinski; the guys who did the Columbine shootings weren't in the "popular" group and had a very small but close group of friends, Richard killed hundreds of people throughout his career as a mafia hitman and he's a quiet one.