Don't really care enough to fight this out too much, but that is the problem with the shows approach imo, it betrayed the books arc for her too much and so this sudden shift is not established enough to be believable to her present character in the show(same goes with what has happened with a lot of characters).
Unlike the books they did a shit poor job establishing that part of her personality. Too busy chasing "My Queen!" fan fic. Power hungry, yes, a willingness to go to extremes to secure her throne, absolutely, cruelty in punishment, yep, foreshadowing with symbolism and lines of dialogue what GRRM likely told them years ago, for sure.... Suddenly going from wanting to instill fear as a ruler to committing mass genocide in the throws of unconditional surrender because you didn't get some dick and the Red Keep triggered you(per dumb and dumber)?
The show needed some better set up is all, but without it it is jarring and I get a lot of the backlash.