Bran is an evil dingus. He gave it away with that obnoxious response “Why do you think I’m here,” thereby accepting the offer to rule the Seven (Six) Kingdoms and win the friggin Game of Thrones.
Granted, there’s a lot you can speculate on with a character whose powers are so poorly defined as Bran’s. So let’s start this conversation with this one assumption: Bran has some limited ability to see the future. The show has been ambiguous on this point, but what sealed it for me was the aforementioned quote, implying he had some forward knowledge that he was going to be nominated as king.
R + L = J has been the show’s major fan theory, and it turned out to be correct! So in our minds it is a huge, Big Deal. Narratively as readers, we expect something to be borne out of that huge revelation. But from a character standpoint there were only 2 people on the show who knew about this, Sam and Bran. Bran has his vision, mistakenly thinking Jon is the bastard son of R + L, and Sam has the information that Rhaegar annulled his previous marriage before marrying Lyanna in secret. Together, they realize Jon is the legitimate Targaryan heir.
Come Season 7. Sam is sad that Dany fried his dad and bro, at which point Bran gently suggests that he tell Jon about his true heritage. Jon, being Jon, effectively tells everyone by talking to Sansa. Sansa tells Tyrion, Tyrion tells Varys, Varys is toast, and Dany now knows that most of her council has betrayed her in some way. Cue dramatic music and crazy eyes as the bells ring in the next episode.
I’m not suggesting that these rounds of betrayals are solely responsible for Dany Floor is Lava-ing King’s Landing. As Tyrion’s murder pep talk to Jon points out, Dany has been on this road for quite a while. But I will argue that this betrayal is the final straw.
Think on what the events would have been otherwise, if Sam and Bran had kept their traps shut. Without a strong rival claim to the throne, there is no one for her counselors to turn to, despite their trepidations. Dany never has her falling-out with her nephew. There is no “Fear or Love” discussion. Dany has much more support as they prepare to beseige King’s Landing, deaths of her bff and dragon son notwithstanding. In this version of events, she doesn’t burn King’s Landing out of desperation, and she rises to the Iron Throne without half of her supporters thinking she’s nuts. So what’s a secretly power-hungry Three Eyed Raven to do?
All it takes is a suggestion to a distraught friend. Isolated and without love, Dany chooses fear.
Let’s discuss, with the caveat that any speculation, without additional material from GRRM or D&D is ultimately pointless, i.e. the best sort of discussion.