I don't actually think that Tuon is being inconsisntant with how she was previously. She was shown as honourable, intelligent but very very much driven by her duty. Now she has the full weight of the Empire, at least what is left of it on her shoulders and she has just had a visit from Dark Rand and she knows that the Last Battle is days away if not closer.

The way I see it she has gone full duty mode and is consentrating on what she thinks has to be done to win the Last Battle. If she can be merciful ie the new king in Ebou Dar she will be but only if it does not interfear with her goal of winning the Last Battle.

So lets take a look a look at the world from Tuon's point of view. Firstly her prophecies say she has to get control of the Dragon Reborn in order to save the world. She has just met Dark Rand is that has only reinforced her view that she needs to get some control over this monster asap. She thinks that the main opposition to the Seanchan taking control over the continent is the White Tower. She also thinks that they are controling Rand. If proof was needed were there not Aes Sedai hanging around with him when he visited her?

Her forces have now gained the ability to Travel and the clock is ticking down to the end of the world. Top priority is to capture and or neutralise as many Aes Sedai as possible to remove the no 1 block on Seanchan control and to weaken the Dragon Reborn so that he can be contained and controled and the world can be saved.