Yeah, it was way too sudden and over the top. Not slamming the acting at all.
I'm tired of seeing people tell me there were all these "signs" Dany was going to go mad. Mostly in the recent seasons I saw people wondering about her parentage, and coin-flipping, but those aren't her actions being judged, it's her DNA. Which is really uncool.
But I could do the same justification with pretty much every character, if the writers had chosen to warp the character at the 11th hour. For instance,
Bran is now actually evil. That's why he had no emotion in saying goodbye to Meera, or seeing Hodor die. He arranged for the Night King to get killed so he could take his place. He didn't warn anyone about the coming annihilation of KL -- he actually warged into Drogon to make it happen, which is why we don't see Dany after the chaos and destruction start.
OK, I don't believe that, but there's just as much evidence for it as there is for Dany going bonkers.
Or Sansa is a new Cersei, plotting for her own power. Again, plenty of evidence to read it that way if the writers had gone there. Or, how about Tyrion plotted all along to become the king in all but name. He was intentionally giving Dany bad advice.
Or, Jon is a depressive, and commits suicide in a shocking finale. There's plenty of evidence to support that's where his head is at.
My point being there wasn't any more evidence of Dany ending up where she did than of any other character getting character assassinated at the last minute.