When Arygos dragged his scarred body over and pulled Dick out of C'Thun's remains, the paladin had already been corroded beyond recognition.
Of course, C'Thun had fared even worse. After the titanic energy erupting from the alternate space inside his body reached saturation, it broke through his restraints. Like a detonated nuclear bomb, it tore C'Thun's body apart with ease.
Although Old Gods could survive after losing their physical bodies in the form of chaotic souls, once golden and blue titanic energy filled the hall, no chaos outside order could possibly continue to exist under those conditions.
That went all the more for C'Thun, who had already been weakened to the extreme, but this did not mean C'Thun was dead.
In fact, on Pandaria, which was still shrouded in mists, seven remnants of Y'Shaarj, the strongest Old God, who had been killed tens of thousands of years ago, still survived. Old Gods would never truly die, but at least this time, Dick's life-risking gamble had indeed shattered all of C'Thun's consciousness.
Even if the chaotic energy he had left in this land gathered and took shape again, it would only form another monster. He would never again have the chance to become a true Old God.
So in theory, this blow delivered using the prison's own power had indeed killed C'Thun.
They had accomplished something even the Titans had failed to do, though this was also because C'Thun had courted his own destruction by abandoning his connection to Azeroth. Otherwise, even if they had possessed the strength to kill an Old God, Dick and the others would not have dared risk harming the planet itself to kill a weakened Old God.
From this perspective, Deathwing might not have had good intentions when he suggested that C'Thun escape his prison. Otherwise,