The more I think about it, the more I get convinced that the execution of Tarlys don't make sense for any of the characters.
Randyll: I would have appreciated more if it was an acceptance that he had done wrong by choosing the wrong side and betraying everything he believed on in order to become Warden of the South, and so causing so many deaths which result in nothing for him. A bit like the last moments of Ser Allistar Thorn. But here, he looked like he was in love with Queen Cersei, which makes no sense in whatever direction you look at. And even there, when he saw Dickon joining him, it made sense to change his mind. After all, Eddard Stark betrayed his honor to save Sansa, and Eddard was 10 times the man Randyll is.
Dickon: why, just why? Sure, his dad might have made the wrong choice and decided to live (and die) with it, but Dickon why?
Daenerys: she didn't even try to make him change his mind. She didn't even play the loyalty card (Tarly was a Targaryan loyalist during the war). And with Baratheon brothers, Tywin and Ned dead, he is the best general in Westeros. Considering that Daenerys is on desperate need of a general (Daario was superb but she left him in Essos), she could have moved heaven and earth to convince Tarly to join her. It is easy in the battle for the Iron Throne, but in a war between equals (at best), she needs all the assets she can get.
Tyrion: at least he tried, but probably could have tried harder.
Writers: at this stage, only the warriors matter, so I do not get why they killed Tarly (and the actor seemed to be doing a very good job). It is time to chop of the likes of Littlefinger, Varys and other politicians, not the likes of Tarly.
It was a lose-lose situation for everyone, and they still went for it. Is the Night King the only rational person in the show?