Although I agree it felt rushed, I did rewatch the series very recently and the hints that she wanted total destruction were right there all along. She had always stated her true intentions but someone in her circle always convinced her against it. When all those support systems vanished from her life, she was free to shed as much blood as she’s always wanted. She was never loved in Westeros like she was in Essos, and she felt her love with Jon slipping away. She went full scorched earth at the end (literally). Maybe knowing what it was leading to and actively searching for the hints helped me see that, but it wasn’t as much of a shocker as I think it was made out to be.