While no one would describe 1947's Kiss of Death as a horror movie in the traditional sense, Widmark's embodiment of the giggling killer Tommy Udo established him "as the most frightening person on the screen," according to critic David Thomson in his book The Biographical Dictionary of Film (as quoted by Aljean Harmetz in The New York Times). David Hudson at GreenCine Daily has a good roundup of coverage, and he points to an AP story in which Widmark is quoted as saying: "I played the part the way I did because the script struck me as funny and the part I played made me laugh. The guy was such a ridiculous beast."
Very often, modern horror villains are hidden under masks or gruesome make-up. When they are revealed in the flesh, they're seldom menacing until they pick up a weapon. (Example: Tobin Bell as Jigsaw in the Saw films is not scary in person; it's his fiendish devices that freak out his victims.) Without ever resorting to camp or overly emotive acting, Richard Widmark could be frightening even in straight dramatic roles. Even in the melodramatic Judgment at Nuremberg, which I saw again recently, he was the best, most authentic actor, playing a military lawyer seething with righteous anger. The only modern equivalent I can think of is Daniel Day Lewis' monstrous performance in There Will Be Blood.
But it is Widmark's portrayal of Tommy Udo in Kiss of Death that still sends chills down the spine and defines how horror villains should be played.