Why do you and others always throw this out? I'm not a writer. As a member of the audience though, I know when I'm not convinced that she had good reason to kill thousands of innocents. It seemed disingenuous to the character they've portrayed for 7 seasons and 3 episodes.
Same with Jamie. Show him having some moral compass and breaking from Cersei's toxicity just to have him back in her arms in 1 episode? Did he not know they were probably going to kill Cersei when they all left Winterfell to fight her?
Again, I'm cool with her doing it, just lead up to it in a convincing manner.