The thing about the Valonqar thing is that prophecies are generally fulfilled in an ironic way. Oedipus runs away from home to avoid killing his father and instead meats his bio dad on the road and kills him in a stupid argument. Would anything in Macbeth happened if he didn't kill the king?
Random, tragic, ironic, that's how prophecies go. Trying to prevent it from happening ensures it's fulfillment. The person who hears it is always wrong about who it refers to.
So clearly it can't be Tyrion, it has to be Jaime that kills her. But I also think it's wrong to think that Jaime is going to strike Jaime in righteous fury to dole out some punishment for her sins. He is not pulling up to her like this
Ultimately Cersei has to doom herself but not with just general immoral actions but by actively trying to prevent the prophecy from being fulfilled. For example, by trying to kill Jaime. Maybe even succeeding. The point tho is that she wouldn't die, if she didn't try to stop it.











