Bellamy poisons his sister - The 100
Absolutely no one saw this coming. Bellamy's devotion to his sister had no limits even when Octavia wasn't always worthy of it. No matter how she treated him, Bellamy's desire to protect her above all others never wavered. But Bellamy was finally pushed to his limits, and when Octavia decides to execute Clarke he uses Monty's algae to put her in a coma not knowing if she would ever wake back up.
Matt turns against his own team - Shades of Blue
In a sea of confusion and betrayals, Matt Wozniak ends up murdering a member of his own team Detective David Saperstein. There was quite a bit of miscommunication and clearly, the results were tragic.
Sansa and Arya kill Littlefinger - Game of Thrones
In a shocking turn of events, Sansa goes from plotting her sister's downfall with Littlefinger to letting Arya execute him instead. It was clear from Littlefinger's face that he did not see that one coming.
Eve stabs Villanelle - Killing Eve
There's a lot of things that could have lead to Eve making this decision, but it's clear that even after Eve stabs Villanelle that she doesn't want her to die. Immediately after she pulls the knife out, Eve panics and rushes to stop Villanelle from bleeding to death.
Chuck trades Blair for his hotel - Gossip Girl
This was one betrayal that turned some people (myself included) off of the CW couple. Chuck manipulates an oblivious Blair into sleeping with his uncle in exchange for The Empire, but she stops when Jack reveals that Chuck had been secretly pushing her to do it the entire time. Chuck later tells her that he did what he had to do to win.
Chloe lies on the stand - 13 Reasons Why
In order to make a stronger case against Bryce, Chole promised Jessica that she would tell the truth about her own sexual assault. But when she looks into her boyfriend's eyes from the stand, Chloe just couldn't do it. She leaves both the audience and Jessica wondering why she lied to protect someone who hurt both Jess and herself.