M...... staring Peter Lorre? I saw it as a kid as a B movie back in the late 40’s. If I remember correctly M meant Murder, but wasn’t there a young girl involved? Maybe it meant molester too?
The original “M” with Peter Lorre was a 1931 German classic, directed by Fritz Lang. I thought it was in the public-domain, as worn copies used to be all over the place on VHS, but I’m surprised if there hasn’t been an official high-quality, ‘restored’ release.
There was also a rather forgotten (American) remake, from 1951, featuring David Wayne and Howard da Silva. Maybe you saw that one at the theater.