"Heading Home" by Ramsey Campbell

First appeared in Whispers. Reprinted by Gerald W. Page, Stuart David Schiff, Al Sarrantonio, and Martin H. Greenberg.

Summary:

A scientist discovers not only has he been cuckolded but he's also had his muscles cut by her literally butcher boyfriend. He struggles for every inch up the stairs. The boyfriend hears something and says he's going to investigate.

Spoilers & Commentary:

What a perfectly horrid title pun, and what a perfectly executed mad scientist tale.

The scientist has killed kids in order to make himself immortal. Of course, that doesn't negate what the lovers have done, but Campbell has beautifully switched our loyalties. The boyfriend is called back, and the scientist makes it back to the lab, where his body is. He'll sew himself back together.

I'm a little dubious about the head's propulsion system, though.

The mad-scientist perfection is how he justifies his misdeeds and allies our sympathies until we truly learn what sort of man he is.