And then he’ll see you again, and again, and AGAIN:

 

Before I Hang (1940), in which Boris Karloff plays a doctor who becomes a serial killer when he has himself injected with an experimental serum made from the plasma of an executed murderer…

The Devil Commands (1941), in which Boris Karloff plays a doctor who becomes obsessed with transdimensional telepathic communication when his wife is killed in an auto wreck…

The Man They Could Not Hang (1939), in which Boris Karloff plays a doctor who gets himself resurrected in order to take revenge on the people who got him unjustly executed for murder…

and…

The Man with Nine Lives (1940), in which (you guessed it) Boris Karloff plays (you guessed it again) a doctor who accidentally freezes himself and several of his enemies for ten years using an experimental cryogenic technique, and picks up the feud where he left off when a second doctor finds and defrosts all the preserved bodies.

 
And in addition, because I just couldn’t bear to do an update devoted solely to turn-of-the-40’s mad doctor moves, I also give you:

 
I Am Legend (2007), in which Will Smith is not only much better than Charlton Heston, but is nearly as good as Vincent Price…

Let the Right One In (2008), in which the Swedes show us all the correct way to do an adolescent vampire love story…

and…

The Toolbox Murders (1977), in which a guy with a toolbox commits a whole bunch of murders.