In Game of Thrones'Season 6 finale, Cersei Lannister (Lena Headey) got what she always wanted but never could attain: to be the boss. After her son King Tommen's (Dean-Charles Chapman) suicide, Cersei was proclaimed First of Her Name, Queen of the Andals and the First Men, Protector of the Seven Kingdoms. It was an unexpected turn of events that has left fans scrambling to figure out what comes next.
The only thing certain is: whatever it is, it won't be good for anyone.
Cersei now has nothing to lose. The only thing that kept her tethered to any kind of humanity was her love for her children, and now they're all dead.
She's starting to show some Mad Queen tendencies. Mad King Aerys Targaryen loved to burn people alive, and she used his leftover wildfire to blow up the Great Sept of Baelor and kill her enemies the High Sparrow (Jonathan Pryce) and Maergery Tyrell (Natalie Dormer). She then brutally tortured Septa Unella (Hannah Waddingham), who had beaten and shamed Cersei when she was the High Sparrow's prisoner, by wineboarding and taunting her before leaving her to the Mountain (Hafthor Julius Bjornsson). It was an act of cruelty fueled entirely by vengeance.
Cersei isn't supposed to be the Queen. She seems to have unilaterally crowned herself due to a power vacuum in the chaos of post-explosion King's Landing. No one was around to oppose her, and the people of King's Landing who hate her were willing to put their reservations aside to allow her to ascend, just so someone would in charge during this frightening time. But her reign will be brief. Even Lena Headey thinks so.
Because there's a prophecy to fulfill.
At the beginning of Season 5, the witch Maggy the Frog prophesied young Cersei's future and predicted that she would marry the king; be replaced by a younger, more beautiful queen; have three children who would be crowned but die; and be strangled by her little brother. Everything in that prophecy has so far come true, so there's no reason to believe that the last part won't.
Her brother/lover Jaime Lannister (Nikolaj Coster-Waldau) is going to kill her.
Jaime is the younger twin, and (unlike Cersei) is willing to do things that aren't entirely self-serving. Cersei is not fit to be the Queen, and Jaime knows that. He'll have to remove her. She gave him his gold hand, so it would be a full-circle moment for him to strangle her with it. He's already the Kingslayer, so it's no stretch for him to become the Queenslayer, too.
Of course, it could theoretically also be Tyrion (Peter Dinklage) who kills her - she always thought it would be him, which is part of why she hates him - and Lena Headey told EW that she thinks it will be him or Arya Stark (Maisie Williams) who does her in, but it won't be. Jaime would be more narratively satisfying.
In Season 7, Cersei will be a paranoid, volatile Queen, fighting off threats real and perceived. But the biggest one will come from the person closest to her in the world.