LONDON — It seemed like the perfect pitch to a popular British food magazine that had recently reported on an increase in sales of plant-based food: a new series about veganism.

But the editor of the magazine, Waitrose Food, was not at all impressed by the idea put forward last week by the freelance writer, and he responded with a proposal of his own that led to howls of outrage and his swift resignation from the magazine.

“How about a series on killing vegans one by one,” the editor, William Sitwell, wrote in an email seen by The New York Times. “Ways to trap them? How to interrogate them properly? Expose their hypocrisy? Force-feed them meat? Make them eat steak and drink red wine?

Selene Nelson, the vegan food writer who had sent the pitch to the upscale grocery store’s magazine ahead of World Vegan Day on Thursday, said that while she frequently came across people with hostile attitudes toward veganism, she had “never seen anything like this.”