Disclaimer: I will never own Joffrey Baratheon, or anyone in this universe. This is all GRRM's, not mine.
He drank the cup of wine, making sure every single person saw. He was their King, the True King, and they should fear him. They should fear what he could do. Suddenly, he was awash with something in his throat. It was invading, and so he coughed. It probably was nothing. Until it kept going, and he coughed again.
What had his uncle given him? It was a cup of wine, wine they had all been drinking. There could not have been anything in there. If he survived this, he would make sure that Sansa had his baby in her. He would make sure of it, just as a punishment for his uncle. He was about ready to shout, when suddenly he coughed again. He couldn't stop, it was as if something was there, right at his throat. He started sputtering. No, he was the King, he would not die at his own wedding. He would not die without have taken Sansa's maidenhood, or Margaery's supposed one. He was the King, not Tommen. Tommen was not worthy, he was a little brat.
He could hardly breathe now, and he began clawing at his throat. He didn't care what they saw, he only cared at what he was realizing was happening. His uncle must have slipped poison into his drink. Slowly, he stumbled down the stairs, preparing for the end. He saw the old lady and Margaery, staring at him as if they knew. It was as if they both knew. He tried to point, as if stumble out words, but it wasn't happening. He realized he wouldn't be able to succeed. He was no longer the King, or even a Baratheon. He knew his mother was fucking her brother, his father, but he didn't especially care. He watched as they ran over to him, his mother's shriek the last thing he heard before he felt himself be relieved from his body.