The back of his jacket was "M"d
'tween crooks and the cops he was hemmed
In criminal style
they put him on trial
For biz he was bad, and condemned
Like the "A" in The Scarlet Letter, Peter Lorre's child murderer is marked with an M (Fritz Lang, 1931). The story was based on a true case in Germany, where the underworld helped the police to stop a serial killer because his heinous crimes were bad for business as usual. Here's an earlier M limerick.