I read this short story in a science fiction anthology sometime during the 1960s or 1970s, and the story should have been somewhat older than the anthology.
In this space opera story humans from Earth and the Solar System have spread out accross the galaxy and colonised many planets in many star systems. The humans in the Solar System, the Solarians, have closed the solor system to outsiders for a long time, though some of them travel to other worlds.
The protagonist commands some sort of patrol vessel - military, coast (planet) guard, or police.
A Solarian space ship approaches the border, identifying itself as a hospital ship. The protagonist asks them to be boarded for inspection. The Solarians refuse, and he blasts the ship, destroying it and killing everyone aboard.
The protagonist is court martialed and dismissed in disgrace, but he spends the rest of the story trying to vindicate himself. He risks his lives to investigate the secrets of the Solarians.
And he discovers that some humans in the Solar System have mutated enough to be nonhuman. They may use the unmutated humans for food, and sell the organs they harvest for transplants in other star systems. And maybe they are building a military force to invade and conquer all the other human planets.
The protagonists reveals the truth and clears his reputation.
But I think that facts which he found out later can not retroactively justify the action of blasting the Solarian ship before he knew those facts. Therefore I think that the protagonist should not have been vindicated in the end.
Added 01-30-2022 for future reference.
Clara Diaz Sanchez's answer shows that the story is "Blood Bank", by Wlater MIller, Astounding Science Fiction, JUne 1952.
I remember reading it in a book sometime before about 1980. It might have been All About the Future (1955). There are also collections of Miller's stories; The View From the Stars (1965), The Science Fiction Stories of Walter M. Miller Jr. (1978), and The Best of Walter M. Miller Jr. (1980). I don't recognize enough story titles in them to tell which book it was.