Why wasn't Gregor Clegane sentenced to die after his fight with Oberyn Martell?

Question

At the end of their battle, Clegane clearly confesses to having raped and killed Elia Martell, in front of a large audience.

One would think that such a public statement would be grounds for death penalty. Why did he face no retribution?

Answer 1

He would have been, but he died of poison first.

In the book, Lord Tywin Lannister explicitly laid out his plans for the Mountain to be executed for his publicly confessed crimes:

"Then I fear Ser Gregor may die."

"Undoubtedly. I swore as much in the letter I sent to Prince Doran with his brother's body. But it must be seen to be the sword of the King's Justice that slays him, not a poisoned spear. Heal him."

_{-- A Song of Ice and Fire, book 3 part 2: A Storm of Swords: Blood and Gold}

After Lord Tywin's death, Cersei at first kept to his command that Ser Gregor be executed. After some persuasion from Qyburn, she allowed him to take the Mountain for his vile experiments, but still ordered his head sent to Dorne upon his death:

"His size," the queen suggested, frowning. "Gregor is a very large man. Also a very stupid one. Too stupid to know when he should die, it seems." She held out her cup, and Senelle filled it once again. "His screaming frightens Tommen. It has even been known to wake me of a night. I would say it is past time we summoned Ilyn Payne."

"Your Grace," said Qyburn, "mayhaps I might move Ser Gregor to the dungeons? His screams will not disturb you there, and I will be able to tend to him more freely."

"Tend to him?" She laughed. "Let Ser Ilyn tend to him."

"If that is Your Grace's wish," Qyburn said, "but this poison . . . it would be useful to know more about it, would it not? Send a knight to slay a knight and an archer to kill an archer, the smallfolk often say. To combat the black arts ..." He did not finish the thought, but only smiled at her.

He is not Pycelle, that much is plain. The queen weighed him, wondering. "Why did the Citadel take your chain?"

"The archmaesters are all craven at heart. The grey sheep, Marwyn calls them. I was as skilled a healer as Ebrose, but aspired to surpass him. For hundreds of years the men of the Citadel have opened the bodies of the dead, to study the nature of life. I wished to understand the nature of death, so I opened the bodies of the living. For that crime the grey sheep shamed me and forced me into exile ... but I understand the nature of life and death better than any man in Oldtown."

"Do you?" That intrigued her. "Very well. The Mountain is yours. Do what you will with him, but confine your studies to the black cells. When he dies, bring me his head. My father promised it to Dorne."

_{-- A Song of Ice and Fire, book 4: A Feast for Crows}

A giant skull was duly sent to Dorne, but it is speculated that

the Mountain was revived by necromancy and is in fact the massive and silent Ser Robert Strong, Cersei's champion.