Today's is one of my favourites, Władysław Mazurkiewicz (1910 - 1957), sometimes called 'Beautiful Władek' or the 'Gentleman Murderer'. He got away with it so long because his victims were members of the pre-war Kraków middle classes who during the war and the Stalin years often went missing, either because they'd fled or because the state had 'disappeared' them. As Time Magazine said at the time: "So many people were removed by the secret police that no one thought of suspecting a private initiative of it."
One of his victims was the landowner and businessman Jerzy de Laveaux (presumably a relative of both General Ludwik de Laveaux and the artist of the same name) who he killed during the war. Post-war he also killed de Laveaux's wife and sister-in-law.
Between 8 and 30 victims, killed by either cyanide-laced sandwiches or a shot to the back of the head.
Tomorrow (if time), the infamous (and maybe innocent) Rita Gorgonowa.