The story is "The Rule of the Door" by Lloyd Biggle, Junior.
(I think a novella, probably dating from the 1950's or 1960's) that I read in a collection.
It's classified as a novelette. It was originally published in Galaxy February 1958, and has appeared in several collections. One is the 1967 The Rule of the Door and Other Fanciful Regulations. My copy is in The Silent Sky, whose cover is shown in the link.
The alien hero (a university lecturer)
Professor Skarn Skukarn twisted abruptly on the billowy expanse of his
bed and sat up. [...] His lecture notes lay stacked neatly on his
is tasked by his emperor with collecting a human for his zoo.
He hesitated. His own wrist band tingled sharply, almost painfully.
With a sudden surge of panic he understood what it was that had
awakened him. He bounded away, scrambled back to the lectern to
announce, “To be continued,” pressed the cancellation button, and
hurried off to his own viewer.
The Prime Minister’s face stared out at him, alarmingly pale, haggard,
eyelashes crinkly with fatigue. Skarn could easily guess who it was
that had disturbed his sleep. The Prime Minister scowled and said
enviously, “You are looking well, Skarn.”
“Likewise,” Skarn murmured politely.
“I am not looking well. I am looking miserable. I’m tired.”
“Naturally,” Skarn agreed.
“An Imperial Assignment. You will begin at once.”
“A patrol ship has discovered another inhabited planet. His Imperial
Majesty desires a specimen of the dominant life form for the Royal
The hero explains that there are time-honoured rules about this sort of thing
There is a longish passage where the Prime Minister explains that the Rule of the Door applies. Knowledge of the Rule's content has been lost, so the hero's job is not only to capture an alien life-form, but to "rediscover the content of the Rule of the Door" and "to follow it scrupulously".
and that he can only snatch someone if they are 'evil'.
This is difficult. I don't know whether there are different versions of the story, but in mine, the first explanation of the rule only implies "evil". It doesn't state it. This is the passage which implies it: the hero is describing information he's found after an exhausting search through archives.
“In his inestimable wisdom, the Great Kom realized that the disruption
of the life process of an intelligent being was not a project to be
undertaken impulsively. He formulated a series of maxims: ‘Spare the
humble one, for his nature is sublime. Spare the wise one, for his
nature is rare. Spare the one who loves others more than himself, for
love is the ultimate meaning of life. Spare the head of a family, for
his loss would injure many. Spare the weak, for their weakness renders
them harmless. Spare the generous, for their acts merit generosity.’
There is much more. Some of it I do not understand.”
Explicit mention of evil only comes later, once the hero is on Earth. Dialogue such as the below shows that the hero (i.e. Skarn) is concerned with the attribute:
Afterward, Dork stormed angrily about the laboratory while Skarn
restudied his reports. “The detective agency is in error,” Skarn
announced. “Those men are not evil.”
“They’re evil,” Dork said, “but they’re important. They have positions
of responsibility. The Door may consider that.”
The story continues with the hero's arrival (with an assistant) in some American town, disguised as as a rich, eccentric, immigrant/refugee, eager to get to hear all the gossip
Skarn Skukarn, Jonathan Skarn to the citizens of Centertown, took up
his residence in the new house on a crisp fall day and led a newly
arrived, shivering assistant on a tour of inspection. Skarn’s pleasure
in the house was more than offset by his displeasure with the
assistant. The squat, ill-tempered Dork Diffack was grumpy, insulting
and generally obnoxious.
Skarn organises an "open house", and the "entire population of Centertown and the surrounding countryside turned out" for it. "The wooded hill was packed with cars, the highway was lined with parked cars, and the State Police had to call in reinforcements to keep traffic moving."
Gossip isn't explicitly mentioned at this stage, but it's clear that Skarn wants to know about the guests:
Upstairs in the laboratory, Dork disgustedly watched their antics in a
viewer and kept a sharp eye on his humming instruments; and at the end
of the day he announced to Skarn that they had collected sufficient
the prime contenders for the distinction are: the mayor, the police chief, a local car mechanic, and the town drunk!
He read the four reports again. The Honorable Ernest Schwartz, Mayor
He turned to the next report. Sam White, Centertown Chief of Police.
Jim Adams, the Centertown drunk.
Elmer Harley, a ne’er-do-well mechanic. A good mechanic, it was said,
when he worked at it.
I'll stop here, for the moment anyway, but the above is proof that this is the story. You have an impressive memory!