I don't agree with mad Daenerys for roasting the Tarlys. It reminds me of when Ned Stark cut the man's head at the start of the show. Sometimes you need to do unpleasant things, and she did give them a way out.
Dany roasted the Tarlys for one very simple reason - the plot needed it to happen in order to set the seeds for Sam to be opposed to Dany.
My argument is:
The books and earlier seasons on which they were based were written ‘forwards’. In other words a character would do something which would lead to this consequence and they would react in this way. This would lead to another consequence and so on. The failure of Robb’s campaignn can be put down to a series of tiny missteps by multiple characters which amplified as time went on. It is part of GRRM’s ‘gardener’ writing style which has probably led him into the dead end he is in now. Either way, characters do believable and justifiable things according to their situation which lead to consequences.
Since the end of season 4 and the breakdown of working relationship between D&D and GRRM the writers have been writing the series backwards. They are working to a series of bullet points of ‘things that must happen’. They then write the series, episodes, events and dialogue based on ticking off the ‘events’ to lead to the end point.
The consequences of this is that characters act in ways that serve the plot regardless of their character regardless of how inconsistent this makes them appear. Arya’s character particularly suffered last season as they tried to shoehorn her into many different plots and their outcomes.
Dany roasting the Tarlys serves one particular plot; Sam is the Kingmaker for Jon.
Why would Sam be against Dany despite the fact that his best mate is in love with her and she has a huge army and dragons? Because she killed his family and shows no remorse.
Forget the fact that Sam hates his father, forget the fact that Jon has executed traitors without a shred of argument from Sam, forget the fact that Tarlys were traditionally Targaryan loyalists.
This plot can be traced back to season 6 when they first introduced the Tarlys and Hornhill to the series. Why bother introducing them in the first place? Sam has no desire to be lord of Hornhill, and having Heartsbane is of no consequence now that Gendry can ‘forge’ dragonglass into almost any weapon. Simply, they were introduced in order for Dany to execute them in order to piss Sam off.
The same plot development applied to the ‘mission beyond the wall’ last season. It had to happen to the Wall could fall and a dragon could be lost. However, the rationale behind the mission was ridiculous, stupid and poorly executed and, miraculously, ended up with only red-shirts and Benjen Ex Machina dying.
In other words, characters doing something is no reflection on their character. They are doing it in order to serve a purpose for some future plot point. Trying to analyse character motivations and actions at this point is an exercise in futility. Just enjoy the visuals.
I’m just smug that (for the first time) I made a prediction about the series that ended up being right.