[Warning: This post contains spoilers for episode 4 of season 7 of HBO's Game of Thrones. If you're watching the show, catch up already!]
Last week's Game of Thrones episode has everyone guessing what's going to happen, ultimately, between Daenerys Targaryen, mother of dragons, and Cersei Lannister, mother of no one. Dany has the backing of the Dothraki army, three massive fire-breathing dragons, and an imperviousness to fire, while Cersei may be the coldest bitch television has ever seen. Either woman is tough enough for the job.
We have seen Cersei's Modus Operandi over and over again. She makes a short term play that costs her in the long run.
Tycho Nestoris of the Iron Bank said some very revealing things this episode. One was that the Bank does not Bet on winners and losers, it instead invests in the chosen victor. He also says the bank will support the Iron Throne once their debt is paid, not necessarily the current Queen, but whoever is on the throne. Second he appears to be flattering her by saying Tywin never paid his debt in full and she is really outshining her father by doing so.
This is her big mistake, Tywin was no fool and he knew that while the Lannisters were in debt to the Bank the Bank had a vested interest in their success. By paying the debt in full Cersei has allowed Tycho to wash his hands of the Lannisters altogether. After what we saw on the battlefield we have a good idea whose position is strongest and who the Bank would like to back. This clearly contributes to the dire situation in Kings Landing with no grain and limited supplies. She will not Abdicate, instead force the dragons to destroy the city at which point the Kingslayer will be the Valonqar most assume he will be. The irony of this theory is in this case the Lannisters are undone by paying their debt.
This theory totally makes sense, and a lot of Reddit agrees (you can head on over there and check out the conversation). But basically it boils down to the fact that while the Lannisters still owed a lot of gold to the Iron Bank, the bank would have a vested interest in her success. But when Cersei repaid the bank the full amount owed, she sealed her fate—now the bank has no reason to back her and try to keep her alive.
And if the bank has to choose between Cersei and Dany, who do you think they'd go with? The woman currently trapped in her castle and quickly running out of supplies, or the woman not trapped, who just took out a large chunk of the Lannister army in a matter of minutes with her seemingly indestructible fire-breathing dragon? So, yeah.