In the '80s movie From Beyond, scientists create a resonator that allows them to see creatures from an alternate reality. In the opening scene, the sadistic Dr. Pretorius is beheaded by a strange creature. My question is why did he become a member of the alternate universe instead of just dying?
My interpretation was that eating the brain of Dr. Pretorius caused the creature to absorb the Doctor's memories and personality, similar to the planarian worms that supposedly absorbed memories of other worms that were ground up and fed to them, or the worms that ate the bodies of magicians in another Lovecraft story, The Festival, and learned to walk instead of crawl. Note that later on the same creature (with Pretorius' personality) twisted off Crawford Tillinghast's head and ate it. The creature then had the personalities of both Pretorius and Tillinghast, who battled for control of the creature.
It seemed obvious to me what was happening but finding an official source is turning out to not to be so easy.
The Resonator and unnecessary assumptions
The Resonator in From Beyond stimulates a subject's pineal gland, thereby "opening a door to a parallel (and hostile) universe", as described here.
As the OP points out, there seems to be contradiction here: Dr. Pretorius is decapitated and his head is consumed by a ghastly creature; yet later he is seen alive.
However, it is probable that there is no contradiction. There is no reason to assume that Dr. Pretorius survived, and there is no reason to assume that the severely disfigured Dr. Pretorius that appears when Tillinghast and McMichaels turn on the machine again is the Dr. Pretorius of their universe.
Possibility #1: Parallel universe = Parallel Pretorius
It is the parallel Dr. Pretorius of the parallel universe who appears to McMichaels and Tillinghast when the doorway between realities is reopened. This is a violent and twisted version of Pretorius, consistent with the overall nature of the parallel universe.
That being said, this possibility doesn't take into account the shapeshifting nature of the Pretorius creature. That brings us to Possibility #2.
Possibility #2: Shapeshifting brain eater
There is also the possibility that it was not a parallel Dr. Pretorius but rather a shapeshifter or Shoggoth from the alternate universe that incorporated Pretorius' consciousness when it killed him. The Pretorius creature may have done the same to Tillinghast later in the film, when it ate Tillinghast's brain.
Further Support for Possibility #2
In 2000, Andrew Migliore and John Strysik wrote a book called Lurker in the Lobby: A Guide to The Cinema of H.P. Lovecraft, available online here. It provides further evidence that the creature with Pretorius' appearance is something different from Dr. Pretorius himself:
Windows break again and the supposedly dead Pretorius appears naked in the resonator's shimmering red light. But Crawford is a Doubting Thomas and reaches out to touch the doctor's skin — skin that is actually a mass of sickly soft bio-putty. Pretorius mutates before their eyes and Crawford quickly shuts the Resonator down, exclaiming, "That will be enough of that!"
In other words, the Pretorius creature does not appear to be human; rather, it seems to be formed from a kind of gel ("bio-putty") that can be molded into different forms.
Possibility #3: Hallucination
I like this one because it is the most controversial. The Resonator doesn't actually open the door to an alternate universe. It just causes intense, disturbing hallucinations. Tillinghast really did kill Pretorius and the Dr. Pretorius we see later is a hallucination caused by the reactivation of the Resonator. This is not a popular theory, but a possibility nonetheless.
This aspect of the film is explored a little further here.
In any case, the real Dr. Pretorius is almost certainly dead. The creature with his form is not "our" Dr. Pretorius, but any one of a number of Lovecraftian creatures — a twisted doppelgänger of the human Pretorius we meet at the beginning of From Beyond. (That "creature" could even be Tillinghast's own pineal gland, flooding his mind with dangerous hallucinations.)