"The Dance of the Changer and the Three", a short story by Terry Carr, first published in the 1968 original-stories anthology The Farthest Reaches edited by Joseph Elder; the reprint in Vertex, April 1973 is available at the Internet Archive. Does any of these covers ring a bell?
The premise was that we had a colony of sorts on an alien planet, that was comprised of a team of workers. These workers were killed,
After we'd been on the planet for four Standard Years, after we'd established contact and exchanged gifts and favors and information with the Loarra, after we'd set up our entire mining operation and had had it running without hindrance for over three years—after all that, the raid came. One day a sheet of dull purple light swept in from the horizon, and as it got closer I could see that it was a whole colony of the Loarra, their individual colors and fluctuations blending into that single purple mass. I was in the mountain, not outside with the mining extensors, so I saw all of it, and I lived through it.
They flashed in over us like locusts descending, and they hit the crawlers and dredges first. The metal glowed red, then white, then it melted. Then it was just gas that formed billowing clouds rising to the sky. Somewhere inside those clouds was what was left of the elements which had comprised seventeen human beings, who were also vapor now.
and someone was sent to the planet to understand why these workers were killed.
Not exactly. The narrator, having survived the attack, tries to get an explanation from the natives:
I sent out a sequence of lights and movements that translated, roughly, as "What the hell did you do that for?"
And Pur glowed pale yellow for several seconds, then gave me an answer that doesn't translate. Or, if it does, the translation is just, "Because."
Then I asked the question again, in different terms, and she gave me the same answer in different terms. I asked a third time, and a fourth, and she came back with the same thing. She seemed to be enjoying the variations on the dance; maybe she thought we were playing.
Well . . . We'd already sent our distress call by then, so all we could do was wait for a relief ship and hope they wouldn't attack again before the ship came, because we didn't have a chance of fighting them—we were miners, not a military expedition. God knows what a military expedition could have done against energy things, anyway. While we were waiting, I kept sending out the "eyes," and I kept talking to one Loarra after another. It took three weeks for the ship to get there, and I must have talked to over a hundred of them in that time, and the sum total of what I was told was this:
Their reason for wiping out the mining operation was untranslatable. No, they weren't mad. No, they didn't want us to go away. Yes, we were welcome to the stuff we were taking out of the depths of the Loarran ocean.
And, most important, they couldn't tell me whether or not they were likely ever to repeat their attack.
The aliens then proceeded to tell a story, in order to explain to this human their culture and why these workers had to be killed. The name of this story/myth was also the name of the short story.
Actually, the narrator had learned the story well before the attack. It starts like this:
This all happened ages ago, and in the depths of space beyond Darkedge, where galaxies lumber ponderously through the black like so many silent bright rhinoceroses. It was so long ago that when the light from Loarr's galaxy finally reached Earth, after millions of light-years, there was no one here to see it except a few things in the oceans that were too mindlessly busy with their monotonous single-celled reactions to notice.
Yet, as long ago as it was, the present-day Loarra still remember this story and retell it in complex, shifting wave-dances every time one of the newly changed asks for it. [. . .] In fact, you could take this as a piece of pure fiction, because there are damned few real facts in it—but I know better (or worse), because I know how true it is. And that has a lot to do with why I'm back here on Earth, with forty-two friends and co-workers left dead on Loarr. They never had a chance.
There was a Changer who had spent three life cycles planning a particular cycleclimax and who had come to the moment of action. He wasn't really named Minnearo, but I'll call him that because it's the closest thing I can write to approximate the tone, emotional matrix, and association that were all wrapped up in his designation.
When he came to his decision, he turned away from the crag on which he'd been standing overlooking the Loarran ocean, and went quickly to the personality-homes of three of his best friends. To the first friend, Asterrea, he said, "I am going to commit suicide," wave-dancing this message in his best festive tone.
His friend laughed, as Minnearo had hoped, but only for a short time. Then he turned away and left Minnearo alone, because there had already been several suicides lately, and it was wearing a little thin.
The name of this story/myth was also the name of the short story.
I learned a lot about them. Just enough to translate, however clumsily, the wave-dance of the Changer and the Three, which is their equivalent of a classic folk-hero myth (or would be if they had anything honestly equivalent to anything of ours.)
I think the myth was about three of the aliens making a journey, meeting a fourth, and then undergoing some sort of metamorphosis.
They meet the Oldest:
When the Oldest had told them what they wanted to know, the Three came alive with popping and flashing and dancing in the air, Pur just as much as the others. It was all that they had hoped for and more; it was the entire answer to their quest and their problem. It would enable them to create, to transcend any negative cycle-climax they could have devised.