Many years ago, 1960s, I read this great story
"Fessenden's Worlds", a short story by Edmond Hamilton; first published in Weird Tales, April 1937, available at the Internet Archive. If you read it in the 1960s it was probably in the anthology Beyond Time and Space edited by August Derleth, either the big 1950 hardcover or the abridged 1958 paperback.
about a scientist who creates and manipulates microscopic worlds.
"Always," he said, "astronomy has been an academic, impotent science—the one science that merely observes its subject and does not experiment with it.
"I resolved to be the first empirical astronomer. Do you understand that, Bradley? I would not only observe a universe—I would experiment with it."
I stared. "Experiment with the universe? You must be speaking allegorically."
Fessenden shook his head. "No. And I did not say 'the' universe, but 'a' universe. A universe that I have created, for my experiment."
I said impatiently, "That's a grandiose metaphor, but just what does it mean?"
"It's not a metaphor at all," Fessenden answered. "I mean literally that I have created a universe in my laboratory, a universe with thousands of suns and tens of thousands of worlds."
Using a magnifying device he can perturb planets, often inhabited, and see the results, usually catastrophic.
Fessenden smiled. "I assure you that that tiny race and their world are quite real." He chuckled. "No doubt that little folk think that they have reached such a pinnacle of power and knowledge that nothing can threaten them. We shall see now whether or not they are able to face a real danger."
He turned to a curious needle-like instrument and carefully trained it upon that part of the microcosm which held the tiny white spark that was the sun of that world we had been watching.
There was a tiny comet crawling through the swarm, some distance from that white sun. Fessenden touched a knob, and from the needle-like instrument a thin, almost invisible filament of force crept into the microcosmic swarm of sparks and touched the comet. It seemed to veer a tiny bit aside.
"Now watch," said Fessenden with interest, "and we shall see just how great is the power of that little people."
I did not understand, but I looked again with him through the microscopes at the tiny world. By now, so swift its development in terms of our time, its cities had become even vaster and were roofed with glass-like shields. Huge aircraft flashed above them.
All seemed peace and progress on that world. Generation after generation ticked by as we watched. Then came a mad stir of movement, a wild scurrying about of the little folk, a swift change in the tempo of their life. A faint green light was now falling upon their planet, the baleful glow of a monster comet that was coming headlong toward it.
I knew then that it was the tiny microscopic comet whose course Fessenden had slightly altered. But in the microscope it was colossal, a huge orb dripping green light across the heavens as it rushed toward that world. Remorselessly it came on.
Then that comet struck the planet, and I saw the doom of the little folk's cities. The meteors that were the comet's only solid substance shattered the glass-roofed cities to ruin. The poisonous gases of the rest of the comet veiled that whole world in a toxic cloud.
The comet passed on as we watched, but its deadly gases had wiped away all life from that planet. It was still and brown and dead, now, a lifeless world circling its sun. The ruined cities melted swiftly down into decay and disappeared, as we watched.
Fessenden nodded interestedly. "You see—their knowledge was not enough to save them from the mere slight shifting of a comet's course."
If I remember he shows this to an assistant who recoils in horror that he is causing wanton destruction of inhabited planets.
Yes. Not right away, but after a few more of Fessenden's destructive experiments:
"Fessenden, no!" I cried. "My God, it makes me sick to think what we've been doing here tonight. Those worlds and peoples we've been experimenting with—they're real!"