The real tragedy of the book to show conversion is that the sand snakes actually wanted to crown Myrcella, not kill her.
See, Dorne is the only kingdom that treats women as if they’re equals. Women inherit lands and titles there. In fact, Doran actually has a daughter, Arienne, who leads the sand snakes in this plot. Oh, and she’s the crown princess of Dorne. Trystane is such a minor character.
They want to crown Myrcella, because according to Dornish law, she should have rightfully become the next ruler of Westeros instead of Tommen. Their revenge on Kings Landing isn’t to kill another Lannister, it’s literally to create a civil war against patriarchy. To make the Lannisters chose a side.
So by erasing Arienne, and making the sand snakes entire plot revolve around wanting to kill an innocent girl, the show really destroyed one of the best plot lines and kingdoms in the books.