He's definitely going to kill Tommen. Tommen is probably going to side with Margaery / the sparrows over Cersei and there'll be some ultra dramatic moment where he glitches out and cleaves that fucker completely down the middle. Probably during the Clegane bowl. Then when the hound emerges the victor, he's going to choke Cersei to death
Cersei: Will the king and I have children?
Maggy: Oh, aye. Six-and-ten for him, and three for you. Gold shall be their crowns and gold their shrouds, she said. And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you.
Valonqar is High Valyrian for "little brother". It is the word used by "Maggy", a maegi and fortune teller, in her prophecies to Cersei Lannister and Melara Hetherspoon.[1]