From your description I am pretty sure this is: Christopher Anvil's "The Kindly Invasion" (1966) re-printed in The Trouble With Aliens.
In it the protagonist, Mr. Peabody doesn't trust aliens bearing gifts (the Shaloux) including a wonderful serum as inoculation against all sorts of human ills including keeping everyone young.
As a weapons manufacturer, he develops (and essentially gives away for $37.50 what would be normally $199 as a bargain) a powerful rifle to anyone that wants it which is capable of killing the aliens. "The .30 Recoilless Repeating Sharpshooter"
At the end of the story we find:
The ex-prisoners were glancing at each other. "Well," said one of them, "we must end this resistance to our wise Brothers from the Stars. Did anyone notice how thin that hull was?" "Yeah. Half-a-dozen good men, with the Super-V steel-jacketed ammo, in Peabody rifles. Miss Burell was looking around blankly, "Didn't anybody fall for that Shaloux?" Peabody, despite his aches and pains, was starting to feel better. "Consider," he said, "the Shaloux didn't kill people at random. Instead, they baited a trap, which only attracted certain people. What do you suppose they've done to the human race?""You mean, they've carried out something like natural selection, only faster?""Exactly. And who do you suppose they've selected as survivors?"