Why does everyone complain about her flip?
She didn't flip. It's been a slow decline over several seasons. Ever since she started breaking chains, her advisor characters were on at her to be careful she didn't become the tyrant herself... Now that Jorah is dead and Missandei is dead and Varys is dead and anyone else she'd have listened to is dead, she just carries on descending into the blatantly obvious and inevitable...
Perhaps snap would be a better word? I fully agree with the above but in either case it's a jump from trying to take the city to razing it out of spite.