And all of those counselors are either dead or no longer trustworthy (Tyrion).

The one big change, though, was that her cruelty was always directed at nobles rather than common people. She always sought to punch "up" rather than down so it is weird to see her lash out at the common people.

I can understand her having an emotional breakdown due to her sense of loss and lack of support by Jon, Tyrion and the people of Westeros (the North wasn't very welcoming so it would hard for her to expect the rest of the country would be better). But lashing out at common people is a big change in personality for her.

The only way I could reconcile it is if, right as she finally won what she's sought for so long, she realized it was pointless. There's no love for her in Westeros (literally and figuratively)... only fear... and the first love she knew was with Khal Drogo.

In the House of the Undying, she saw a destroyed throne room and Drogo and her unborn son. Drogo said he was waiting for but she said she still had to meet her destiny. Maybe the massacre was her deliberately making part of the come true while courting her own death.