He came to King’s Landing officially to be the Martell family’s representative at the royal wedding of Joffrey and Margaery, but he quickly revealed his real reason for showing up in town was that he was looking for revenge against the Lannisters for the rape and murder of his sister during Robert’s Rebellion. So with Oberyn now appearing to work with Tywin Lannister by agreeing to act as a judge in Tyrion Lannister’s trial in exchange for a role on King Tommen’s small council, what is Oberyn really after?
“At the end of the day being on top is what's important at any cost [for the Lannisters],” Pedro reveals in an interview with GQ. “Oberyn definitely has some bad blood with them, so to bring him into [Tywin’s] inner circle — in order to continue moving his own agenda forward — is so typically Tywin. Oberyn participates because for him it's a game. He's enticed not so much by being in a position of power, but by getting closer to his enemy. The nugget that Tywin presents to him is the opportunity to face The Mountain.”
So Oberyn is simply playing the game, but he’d better be careful. When you play the game of thrones, you win or you die!