Looking back, she's rally been on this path for a while. Her insistence to Jon not to tell anyone about his parents. All the way back to when she burnt Sam's father and brother for not bending the knee, she's shown signs of "Mad Queen."
Probably longer than that really. I mean, think about her reaction to Drogo pouring molten gold over her brother. I know she hated him, but he was her brother and she seemed kind of happy about it in the end. She also burned the witch who poisoned Drogo and stole her child. It seemed like reasonable revenge at the time, but in retrospect, maybe it was a bit much to burn her and maybe it was a hint regarding how much she loved to burn people. Then she burned several in Mareen with questionable justification. And, she was willing to burn all of the Dotkraki Kahls at Vaes Dothrak. I mean, she could have just escaped, but instead, she decided to burn them all so that she could claim the Dothraki hoard to further her own interests. And it was likely really against the interests of the Dothraki who she planned to tame when she got to Westeros.