The potential for Dany to become the villain was always there but the show decided to take the potential and be like "see this explains why she is killing men, women and children" without ever taking us on that journey.Although I completely understand why people are mad at Mad Queen Dany I think there's a bit of unfairness on it
The writers definitely blew it BUT
I think there's a macro reason why Dany's change appears to be "out of the blue"
Dany throws nikkas off guard because they're so used to female characters talking and behaving the way she did, which was GRRMs point in writing Dany in the first place. She's meant to be a subversion of the type of characters girls and women love to read about
With nikkas guards down they failed to discern that Dany BEEN an a$$hole since at least season 3she became increasingly indulgent and selfish. And she had some cases that justified her anger. But you don't get a nikka back like THAT
A good trick to notice in fiction is when a character starts going "I I I,me me me. I am this. I'M gonna do that" Characters that talk like that become VILLAINS my nikka. Juxtapose that with Jon, the reluctant hero. Another common trope
D&D's selfishness and eventual laziness ruined the Dany arc, but her becoming what she was wasn't as "out of the blue" as nikkas think
They basically went from showing that the seed was there for an Evil Dany to giving us Evil Dany without letting that shyt grow organically.
And that goes back to the main reason why GoT took a bit of a nosedive in later seasons: In their heads, David Benioff and D.B. Weiss were done with working on the show by Season 5 but were forced to finish out their contract with HBO and they treated that shyt like people treat their jobs after giving their 2 weeks notice...they stop giving a fukk and were just counting the days until it was over and they could move on to do the new stuff they were actually interested in doing.