Solstice Massacre

"What do you know about Komarr?" His tone added, An ignorant Betan.
"Just what everyone knows. It was a worthless ball of rock your people annexed by military force for command of its wormhole clusters. The ruling senate surrendered on terms, and were murdered immediately after. You commanded the expedition, or . . ." Surely the Vorkosigan of Komarr had been an admiral. "Was it you? I thought you said you didn't kill prisoners."

―What everybody knows about Komarr

The Solstice Massacre was a war crime that took place during the Barrayaran conquest of the planet Komarr. The two hundred Komarran Counselors that made up the planet's legislature surrendered on terms to Admiral Aral Vorkosigan and were held in a gymnasium in the capital dome of Solstice. On the orders of Vorkosigan's political officer, they were shot by Barrayaran security forces. Rebecca Galen, Ser Galen's sister and Duv Galeni's aunt, was among the Counselors killed.

In response, Vorkosigan personally broke the political officer's neck with his own hands. He was tried for treason for it, and eventually demoted to Captain. The incident earned Aral Vorkosigan the name "Butcher of Komarr", and was one of the reasons behind the later Komarran Revolt. A Massacre Shrine was built in Solstice at a later date to honor the dead.

Vorkosigan for the rest of his life bemoaned the "messy political legacy" left by the Massacre, ruining his up-to-then successful bloodless conquest. The Massacre's lasting effect continued even beyond the marriage of Laisa Toscane to Emperor Gregor, when some Komarran scientists attempted to destroy the wormhole connecting Barrayar with the rest of human space.

Conspiracy theories

"The Komarran oligarchy, supple middlemen that they were, were brought into alignment with that aim, their surrender eased in every possible way. Promises were made, guarantees given; subordinate life and reduced property were life and property still, calculatedly leavened with hope for future recovery. Living well was to be the best revenge all round.
Then came the Solstice Massacre.
An overeager subordinate, growled Admiral Lord Vorkosigan. Secret orders, cried the surviving families of the two hundred Komarran Counsellors gunned down in a gymnasium by Barrayaran Security forces. Truth, or at any rate certainty, lay among the victims. Miles himself was not sure any historian could resurrect it. Only Admiral Vorkosigan and the security commander knew for sure, and it was Admiral Vorkosigan's word that was on trial. The security commander lay dead without trial at the admiral's own furious hands. Justly executed, or killed to keep from talking, take your pick according to your prejudices."

―The conspiracy theory

Many Komarrans believed that the political officer was ordered by Vorkosigan to commit the massacre, then was killed so Vorkosigan could shift blame to the dead officer. Vorkosigan and others suspected the Council of Ministers of giving the political officer instructions to neutralize the leaders of Komarr, to make the populace easier to control; if anything, the Massacre martyred the dead, leading to political unrest for decades, including the Komarran Revolt.