His deserved "Oscar" for "The Lost Weekend" is another example of this dark side - he is supposed to be a writer, but he is a poseur with a serious drinking problem. In fact, he contemplates suicide at the conclusion of the film, only to be stopped by Jane Wyman.
In "Alias Nick Beal" he played his most sinister part (except possibly Tony Wendice in "Dial "M" For Murder"). Here he played Satan, and he is in total control of the game throughout of the movie - the game being politics and power over people. On one level, if one forgets the supernatural elements, "Alias Nick Beal" is as good an abject lesson in the back room deals of American politics as the comedies "The Senator Was Indiscreet" or Preston Sturges' "The Great McGinty". Only here, with violent death thrown in, the seediness of it all becomes more apparent. Possibly the best moment is when the honest, and mostly honorable, Thomas Mitchell is forced to shake hands with Fred Clark, the most notorious political boss in the state. On the other level is the serious attempt to keep some religious allegory in, with people like George Macready (here in a rare good guy part) noting that Beal resembles an ancient picture of the Devil, and that "Los islas de las almas perditas" where Beal comes from means, "The Island of Lost Souls". Religion does play a crucial role in the film, including it's completion.
Leslie Halliwell made the observation that after this film none of the stars ever did as well again. This is not true. Milland did play the evil Tony Wendice, and Macready went on to the mad French general in "Paths of Glory". But more important, Milland kept showing his ability to stretch in the remaining decades of his life. Besides writing his interesting autobiography "Wide Eyed in Babylon", he directed several films, he appeared in several televisions series (one of the few stars who did not fear the new medium - and he was rewarded here too, for in the 1970s and 1980s he was still appearing while many contemporaries retired). Finally he capped his career as the snobbish father in "Love Story". Actually his career is an example of just what can be accomplished if a person is not ashamed to jettison useless or outdated personalities for new ones.