Eh, I feel like both of the character traits transitions were explained its just it was poorly executed and poor writing in Game of Thrones with Dany and largely did not make sense. She was displaying traits she never had. Also it's not even like she went mad like other Targs had. This was just some weird jealousy taken to the extreme that did not track with previous recent actions and logic. They needed more episodes and seasons to develop that sort of change. Not 7 episodes.
In AoT, it's a bit different cuz they have this never explained scientific/mystical system of ppl inheriting the memories of past titan users and the fact that Eren as one of the founding titans gets the entire history of all past users. Then on top of that apparently he saw his entire future and how everything played out.
It's a self fulfilling prophecy sort of thing. Once you get hit hard with pre-determinism and that free will doesn't exist especially when the outcome leads to your main goal you most likely will give in and not fight it. So he basically leaned in to being unsociable due to the truth he wasn't going to reveal and was a callous douchebags so he could insure he was a villain to everyone.
To me with Eren, the whole memories, knowing all the history, and seeing the future explained his change in the end.