The Brain That Wouldn’t Die

From 1962. Wikipedia says: Dr. Bill Cortner is a successful scientist with a beautiful fiancée named Jan Compton. After a horrible car accident decapitates Jan, Dr. Cortner collects her severed head and rushes it to his laboratory, where he revives it and manages to keep it alive in a liquid-filled tray.

Dr. Cortner now decides to commit murder in order to obtain an attractive new body to attach to his fiancée's head. As he hunts for a suitable specimen, Jan begins to hatch some murder plans of her own. Filled with hatred for Cortner because he won't let her die, she communicates telepathically with a hideous mutant in the laboratory cell, telling it to kill the scientist.