I may have encountered this short story in Asimov's Science Fiction magazine. Human explorers encounter an alien spacecraft. Aliens are cautiously friendly, but then tell the humans about an ancient, brilliant and cruel race of beings that conquered and destroyed every civilization they encountered. They were finally defeated and destroyed, or so it was thought, but their home world was never found. It turns out that this powerful, vicious race was in fact, Earthlings.
That's "All the Way Back", a short story by Michael Shaara; first published in Astounding Science Fiction, July 1952, available at the Internet Archive (story starts on page 48); the text is also available here. The aliens call the ancient race the Antha:
"The news of what happened to the Apectans set the Galactic peoples up in arms, but it was not until the Antha attacked a Federation world that we finally moved against them. It was the greatest war in the history of Life.
"You will perhaps understand how great a people the Antha were when I tell you that they alone, unaided, dependent entirely upon their own resources, fought the rest of the Galactics, and fought them to a standstill. As the terrible years went by we lost whole races and planets—like this one, which was one the Antha destroyed—and yet we could not defeat them.
"It was only after many years, when a Galactic invented the most dangerous weapon of all, that we won. The invention—of which only the Galactic Council has knowledge—enabled us to turn the suns of the Antha into novae, at long range. One by one we destroyed the Antha worlds. We hunted them through all the planets of the desert; for the first time in history the edict of the Federation was death, death for an entire race. At last there were no longer any habitable worlds where the Antha had been. We burned their worlds, and ran them down in space. Thirty thousand years ago, the civilization of the Antha perished."
Roymer had finished. He looked at the Earthmen out of grave, tired old eyes.
For more information see this old question.