Fritz Lang's M is available at the Internet Archive, the 1931 tale of a child-murderer (loosely based on Peter Kurten, himself more interesting than any movie we can think of for the sheer number of crimes he got away with before turning himself in.) This is Peter Lorre's first major role, Fritz Lang's first talkie, and still one of the richest films of its kind, plotwise. If you've never seen it, you should probably rent it, if you have seen it, you'll enjoy a refresher from the Internet Archive.

Fritz Lang's M [Internet Archive]
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