English subtitles. It's fascinationg to see parallels to todays movies like "Hannibal" films or "Seven".
I'd say it was definitely the first "killer pedophile" (would that be "pedophobe"?) film. It really is great, and made a star of Peter Lorre. It is truly brilliant, and introduced concepts you still see in serial/psycho killer flicks today - e.g. killer has a psychological quirks, as here the killer has a creepy signature tune he whistles (In the Hall Of The Mountain King) as he stalks his victims. For us non-German-speakers, the action is very easy to follow even if you don't have or don't use subtitles. It is so wonderful to watch a film where not one damn thing blows up, and shots last more than 1/2 second before a cutaway to another shot (as I recently noticed in Casino Royale, which I enjoyed in spite fo the fact the director and editor afflicted the viewers with 3 shots per second in some scenes, the bastards).
For me, one of the neatest facets of M was how the Berlin underworld was presented as a sub-society not all that different from the straight world, with its own set of laws and morality, although skewed from the traditional rules. The crime world in Berlin was supposedly so monolithic and small that everyone in it knew everyone else, and it could control itself by word on the street which was passed almost instantaneously.
Worth a watch. It still staggers me that the world's great classic films are instantly available online and at no cost.
I like how this movie tells a bit about the psyche of the killer. In many modern movies the killer simply remains the monster and any explanations that are given are only used to display him as even more evil, here it is at least stated that he is still human and that he still has some rights, even if the mob demands his death.
I don't think the focus of the movie is the serial killer or if he is responsible for his deeds (that question is never answered). What really gave me the chills wasn't the rapist, but the mob. Note the scenes in the beginning where they try to lynch some old guy for talking to a little girl. Those are completely healthy people who can arguably control what they are doing, but in the mob they as murderous as the psychopath they are looking for. Considering the film was made in 1931 Germany, this amounts to a prophecy.