"In That Instant, a Terrible Surge of Fear Came Over Him at the Thought of Killing His Own"

"The Fastest Draw."
By Larry Eisenberg (1919-2018).

(a.k.a. "The Marvelous Marshal").
Illustration by [George] Schelling (born 1938; HERE).
First appearance: Amazing Stories, October 1963.

Reprints page (HERE).

Short story (9 pages).
Online at Archive.org (HERE).

     "I guess it never can come out just exactly as I want it to."

When there's an empty place in your soul it's hard to fill it . . .

Principal characters:
~ Amos Handworthy:
  "I have a very vivid imagination."
~ Manny Steinberg:
  "Already he was becoming aware of the depression that followed his engineering triumphs."
~ The Marshal:
  ". . . stared at him out of sightless painted blue eyes, his mechanical hand resting 
stolidly on his gun."

Typos: "Many had lost interest"; "the Marshal become's".

Resources:
- "the Mephisto waltz": "The Mephisto Waltzes are four waltzes composed by Franz Liszt from 1859 to 1862, from 1880 to 1881, and in 1883 and 1885. Nos. 1 and 2 were composed for orchestra, and later arranged for piano, piano duet and two pianos, whereas nos. 3 and 4 were written for piano only. Of the four, the first is the most popular and has been frequently performed in concert and recorded." (Wikipedia HERE).
- Unlike many science fiction/fantasy writers, Lawrence Eisenberg actually had scientific training, so it isn't surprising that his best-known character was a boffin:

  "Many of Eisenberg's stories feature his character Professor Emmet Duck-
worth, a research scientist and two-time winner of the Nobel Prize. Duck-
worth's 'bright ideas seem great at first but always end in disaster' with 
one of the professor's many inventions being 'an addictive aphrodisiac 
clocking in at 150,000 calories per ounce—along with a propensity to turn those taking it into walking bombs.' A number of the Duckworth stories 
were collected in Eisenberg's short story collection, The Best Laid Schemes, published in 1971 by MacMillan." (Wikipedia HERE). Also see the SFE (HERE) and the ISFDb (HERE).

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