I, Monster was not a highlight of the late, great, Christopher Lee's career. The movie was a well-meaning attempt by Amicus to cash in on the popularity of Hammer horror films by hiring most of the same cast and crew (Peter Cushing is in this one as well), and then as a special gimmick, shooting the entire film in 3D.

Unfortunately for them, nothing went as planned... Apparently, the rights to Jekyll and Hyde were still owned by another film company, so they had to call the character "Marlowe" and "Blake" instead, and use makeup effects that could in no way be mistaken for those used by Universal or Hammer, so Lee's monster transformation consisted entirely of giving him a unibrow, buck teeth, and a terrible wig. And to add insult to injury, the production team ran out of money for the 3D process partway through the movie, so it's got all sorts of weird gimmick shots... minus the gimmicks. (Lee loves to throw stuff and wave items in front of the camera for no logical reason, and the climactic final fight scene is quite literally obscured by a brick wall)

All in all, a perfect movie for riffing, especially given the popularity of the two Amicus Doctor Who films, made by the same producer.

There's a very blurry trailer here:
https://www.youtube.com/watch?v=LopmHe1wNoo

Or you can watch a slightly less blurry VHS-transfer of the entire movie here:
https://www.youtube.com/watch?v=PR1tc7j01M8

The film is available on DVD, though because it's Amicus, it's probably not in the public domain.