The most breathtaking moment in Season 7 Episode 4, "The Spoils of War," was arguably the closing moments, watching the Kingslayer sink into the depths of the lake, weighed down by his armor. But if Jaime is dead on Game of Thrones, does that mean that Tyrion will kill Cersei? If the series follows the prophecies made in the book, yes. If not, it's a little more uncertain.
In Season 5's premiere, we got a flashback to a child Cersei talking with a fortune teller, Maggy the Frog. According to Business Insider, much of the Maggy the Frog scene was adapted from George R.R. Martin's book A Feast for Crows. Especially the prediction that Cersei would be "queen, for a time" until "another, younger and more beautiful" casts her down and takes all she holds dear and the prediction that all her three children would die.
But just as the scene should have reached the third of the prophecies made in the book, it instead cut back to Cersei in the present on the way to her father's funeral. In the book, the witch states: "And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you." Since valonqar is High Valyrian for "little brother," fan theories have been obsessed with this prophecy meaning that Jaime, not Tyrion, will murder his sister.
While Jaime is his sister's twin, he is the younger twin (also making him a contender for the younger, more beautiful person who takes all she holds dear). And it would be so much more unexpected if Cersei's lover was the one to murder her and not her other younger brother, who has always disliked her. Reddit users like LordVido cite the look Jaime gave Cersei when she was crowned as a sign "he saw the Mad King again" while user kittenkissies alludes to the Kingslayer's golden hand (which, given that the prophecy in both the book and the show describes the death of her children in the line "gold will be their crowns, gold their shrouds") as an example of how "foreshadowing is very heavy handed." Quite literally in this case.
But, if Jaime dies, then Tyrion would be the only little brother Cersei has left. And let's face it, it's looking more and more likely that Daenerys' Hand will be in a position to murder his sister. "The Spoils Of War" showed us all the advantages Daenerys has over Cersei; the Dothraki appear to be vastly superior warriors to the Lannister's armies, while Qyburn's dragon-murdering machine only managed to wound Drogon before he incinerated it.
Besides, we've already seen Tyrion taking out his own father with a crossbow. We also saw him strangle his former lover, Shae, to death. He's clearly as capable of brutal murder as any other character on the show. And, given that Cersei is also meant to die via strangulation, Shae's death could have been foreshadowing.
Still, there's some ambiguity on this front even if Jaime is dead. For a start, there was no indication from the Maggy the Frog television segment that a prophecy had been left out. The witch tells Cersei she can ask her three questions and Cersei proceeds to ask three questions: when she'll marry the prince she was promised to, if she will be queen, and if her and the king she learns she will marry will have children. We didn't get any indication that the witch had more prophecies to give in the show.
But it did cut away to the present before we witnessed the child version of Cersei leave the witch, so there is always the potential that we'll get that one final prophecy right before the queen's death scene. All the same, it's hard to imagine that Tyrion will be the one to kill his sister. It's a little too on the nose in a series that has been all about giving the audience the plot development they least expect, whether that's Theon's abandoning his sister Yara after a long redemptive narrative arc or audience favorites getting massacred at the Red Wedding.
Some Reddit users argue there is one other option provided by the prophecy. Reddit user dremling argues that Maggy's prophecy means Jon Snow will kill Cersei, stressing, "The Valonqar means 'the little brother', not Cersei's little brother." But they also suggest that this doesn't necessarily just mean anyone on the show who is a little brother, because why say it in Valyrian? No, they claim this suggests that the person who murders the Lannister queen will have Valyrian blood, and, since the Targaryens have Valyrian blood, and Jon Snow, unbeknownst to him, is part of that family and "the little brother of Rhaenys and Aegon," (though it's worth stressing that Jon Snow is Rhaenys and Aegon's half brother, not full brother), the smart money could be on him to take her out.
So, sure, if Jaime dies, then arguably Tyrion would be the obvious choice to kill his sister if the book prophecy also applies to the series. But the very fact that the Hand of the Queen would be so obvious implies that it probably won't be the case.