A lot of fans seems to object and argue that Daenerys did not have enough reason to go ahead and burn all those innocent people in King’s Landing, that it’s just D&D f$@&$ing it up again, that they say the hell with motives, and just want to get this over with.
Well, I don’t see it that way.
IMO she have a very good reason (in her own mind), and that reason is fear.
Think what will happen in her POV if she would just stop the war and try to keep her throne – no one really fears her – an evidence to that is the recent betrayals of Varise, Jon and Tyrion.
No one really loves her – Missandei is murdered, Jorah is dead, Jon don’t even want to kiss her for heaven sake!
She realized that the only way she could rule while Jon is a better contender then her for the Iron Throne, the only way to overcome the one that everybody seems to love and appreciate, is if she would inspire such fear and horror in everyone that they would have to obey her and be afraid to subvert and even think about replacing her.
Add to this the abuse she had suffered all her life, abuse that each of you would go mad of inflicted by it, adding to that (or part of it) all her loved ones were murdered and killed, and her horrifying dynasty that genetically inherited her craziness and madness for blood and fire, and you will receive an adequate answer to her decision to burn them all.