Linoge, among others, discusses the glaring idiocy of the latest antigun nonsense statement, "Fact: Every criminal was once law abiding citizen[sic]" which as an argument is a giant logical goose-egg. Every Nazi dictator was once a baby, too, but that doesn't mean we should chuck them all out with the bathwater just to eliminate the possibility of future Nazi dictators.
However, there have been studies done that point out that, except for the rare nutcase who "just snaps" one day or the honest-to-god "crime of passion", the overwhelmingly vast majority of convicted murderers have long criminal records before they ever get put away for homicide. It's a rare killer who first meets the cops over a cooling body; they're often already on a first-name basis with the local constabulary and nobody's very shocked that "ol' so-and-so finally up and shot somebody."
(Among my readers here, there are enough cops, as well as lawyers who play on offense, defense, or special teams, that hopefully somebody's memory will get jogged enough that I can find a link. I first read the study in the mid-'90s...)