Actually I'm quoting RealityCreator above (quoting quotes within posts makes the system go wonky) and I have to disagree with this statement. If Jon had not been a Targaryen and the rightful heir, then Dany might not have gone completely crazy. If Jon had continued to be able to love her, not just as a Queen but has HIS beloved, then maybe, just maybe, she might have held it together at the end -- even in the face of the loss of two dragons, most of her troops, Ser Jorah and Missandei. If Jon had not been the rightful heir, Varys would not have betrayed Dany for the simple reason that there was no other alternative to her. If Dany had not burned the city, Tyrion would not have abandoned her (though she still might have imprisoned him when she learned he freed his brother.) So yeah, I think the reveal to Dany that -- after all she had been through -- she was NOT the rightful heir to the iron throne, I assume THAT was the last straw that pushed her over the edge into Mad-Queen-Burn-Them-All territory.