MAJOR SPOILERS AHEAD
I'm going to begin with stating that Stannis has never been a favorite of mine. I've never liked him. However, if asked before last night if I thought he could kill his daughter I would have said no way. But that doesn't mean that I'm incapable of believing the events in "The Dane of Dragons." He travels with a powerful witch and has used magic before doing whatever it took, including sacrificing people, to win. People are talking about how it's an "infuriating way to ruin Stannis as a character..." (Forbes) I never saw him as some good guy who deserved to be king. I think Stannis is an example of how power and the need to win at all cost can take over drowning out all compassion and reason.
According to the showrunners in "Inside the Episode" it was George R.R. Martin himself who told them how Shireen will die. Exactly how she dies in the books is yet to be seen. (I haven't read any of the books. I came to Game of Thrones through the show.)
Showrunner Dan Weiss told EW what he believes about the fans reactions and the question, "How could you do that?" He points out how Stannis and Melisandre have been sacrificing people all along.
"It’s like a two-tiered system," he said. "If a superhero knocks over a building and there are 5,000 people in the building that we can presume are now dead, does it matter? Because they’re not people we know. But if one dog we like gets run over by a car, it’s the worst thing we’ve ever seen.
"I totally understand where that visceral reaction comes from. I have that same reaction. There’s also something shitty about that. So instead of saying, ‘How could you do this to somebody you know and care about?’ maybe when it’s happening to somebody we don’t know so well, maybe then it should hit us all a bit harder."
On the subject of the show getting boring, maybe it's my generation, who grew up on slower paced shows and movies, but I have never found Game of Thrones boring. There are episodes that are slower paced, but really this show is about the drama. Sometimes drama takes time. I find the same complaints in The Walking Dead fandom. One of the reasons I watch The Walking Dead is for the "boring" human drama scenes. Game of Thrones plays out in a similar fashion.
You can watch "Game of Thrones Season 5: Inside the Episode #9" here: