Years ago I read a Science fiction short story that I really want to track down.
"A Bad Day for Vermin" a.k.a. "The Exterminator" by Keith Laumer, available at Project Gutenberg; originally published in Galaxy Magazine, February 1964, which is available at the Internet Archive. [Warning: this text contains racist language that some may find offensive.]
Any of these covers look familiar?
The quotations below are from the version in the Laumer collection Alien Minds.
Set is a small sleepy, American town. Then the Alien lands in the middle of town and begins in on a speech of greetings to the townsfolk. Before he can get far, he is shot and killed instantly.
"People of the Green World, happy the cycle—"
Heads turned at the clump of feet coming down the side aisle; a heavy-torsoed man of middle age, bald, wearing a khaki shirt and trousers and rimless glasses, and with a dark leather holster slapping his hip at each step, cleared the end of the front row of seats, planted himself feet apart, yanked a heavy nickel-plated .44 revolver from the holster, took aim, and fired five shots into the body of the Fianna at a range of ten feet. The violet form whipped convulsively, writhed from the bench to the floor with a sound like a wet fire hose being dropped, uttered a gasping twitter, and lay still.
The stunned townsfolk whirl to see who killed him, and it is their racist, town exterminator who makes a sneering comment about how "...it's my job to kill vermin..." as he reloads his town-issued rifle.
He used a revolver, not a rifle, and he doesn't seem to be a public employee.
The sheriff raised his hands, his jowls drawn down in a scowl. "What about it, Judge Gates? Any law against Cecil Stump killing the . . . uh . . .?"
The judge thrust out his lower lip. "Well, let's see. Technically—" he began.
"Good Lord!" someone blurted. "You mean the laws on murder don't define what constitutes—I mean, what—"
"What a humern is?" Stump snorted. "Whatever it says, it sure-bob don't include no purple worms. That's a varmint, pure and simple. Ain't no different killin' it than any other critter."
"They, by God, we'll get him for malicious damage," a man called. "Or hunting without a license—out of season!"
"—carrying concealed weapons!"
"—creating a disturbance!"
Stump went for his hip pocket, fumbled out a fat, shapeless wallet, extracted a thumbed rectangle of folded paper, offered it.
"I'm a licensed exterminator. Got a permit to carry the gun, too. I ain't broke no law." He grinned openly now. "Jest doin' my job, sheriff. And at no charge to the county."
To his surprise, a stranger he doesn't recognise emerges carrying his gun. When challenged, he explains that he's the new town exterminator...and that it's his job to kill vermin as he aims the gun at the ex-exterminator.:
Above, the door opened again. A tall man in a leather jacket stepped out, stood looking down. The crowd pressed back. Senator Custis and the reporter moved aside. The newcomer came down the steps slowly. He carried Cecil Stump's nickel-plated .44 in his hand. Standing alone, Stump watched him.
"Here," he said. His voice carried a sudden note of strain. "Who're you?"
The man reached the foot of the steps, raised the revolver, and cocked it with a thumb.
"I'm the new exterminator."