Before I Hang (1940)
In the late 30s/early 40s, Boris Karloff had two basic roles (outside of his work at Universal) - he would either play the kindly misunderstood scientist or the condemned/murdered man given new life. In Before I Hang, he gets to play both.
In the space of 10 minutes, Dr. Garth (Karloff) is sentenced to hang for the mercy killing of a patient, goes to prison, meets Dr. Howard (Edward Van Sloan) and perfects his youth serum. Alas, he makes the all-too-common mistake of using the blood of a murderer, so when he injects himself roughly 15 minutes in, you know troubles a-brewin'.
Well, you might be able to guess where this is going, and you'd be right. Kindly Karloff becomes Killer Karloff, strangling people with his pocket hanky.
The primary problem with this film is that it's so rushed to start out, like the filmmaker's were impatient to get to the good stuff. The reasons for Garth's imprisonment are delivered as a monologue by the man right before his sentencing. We never see this murderer whose blood is used in the serum, but we're meant to understand that Garth's all-new nervous habits are shared with this unwilling donor. Poor Edward Van Sloan is out of the movie far too quickly, which is too bad (even if he did look ridiculous with his hair dyed black).
As usual, the reason to watch is Karloff, who can make even the most polite character seem to be hiding sinister intentions. His personality switch one-third of the way along is subtle, but very effective.
A two-bit script and lousy direction add up to a minor entry in Karloff's filmography. At this point in his career, he was too old (53) to play any make-up heavy monsters and his next great horror role wouldn't come about until his work with Val Lewton. Until then, he was stuck in in middling efforts like this. For Karloff completists only.