Since she set foot in Westeros, the fiction about her coming to undo tyranny has unraveled. She came to conquer and re-establish her rule, even though the collective Kingdom/Houses and people rose up in rebellion due to her family's poor rule and tyranny. So call it delusion, arrogance, whatever, she never came for altruistic purpose; it was about gaining power--and coming with Dragons and Dothraki, it wasn't going to come without slaughter. Its not a sudden turn; she's made a choice that has been hinted at all along.
All of that makes my dick hard