Another glaring moment of a double standard that irks me beyond belief between the Starks and Targaryens is how little difference there is between the Boltons and Tarlys.
Roose Bolton actively worked against Robb with Walder Frey and Tywin Lannister to have Robb, his men, mother, and pregnant wife killed. Roose himself stabbed Robb in the heart and said, "The Lannisters send their regards."
The Boltons killed their chosen king, rightful Lord and Warden of the North, and usurped the Starks position in Winterfell and Ramsay was, well, Ramsay.
Now, moving on to the Tarlys:
Randyll Tarly was a bannerman of House Tyrell, and proud of his loyalty to them despite them having bent to Robert at the end of the Rebellion and being well-known Targaryen loyalists.
Randyll demands to know how Cersei hopes to defeat Daenerys when she has 3 full grown dragons, and when Jaime pulls him aside he says, "I come when my queen commands it, and I know what she does to people who deny her."
Meaning he knows she burned the Sept to the ground. He knows she's behind decimating the House his House has sworn loyalty to for thousands of years, and he even mentions having known Olenna since he was a child. He also expounds on his absolute loyalty and how House Tarly aren't oathbreakers, that they don't stab people in the back.
But then Jaime says, "Cersei will need a new Warden of the South once the war is over."
And Randyll jumps on that immediately. He actively agrees to betray Olenna and fight against Daenerys. All for a title and new, pretty castle.
This is Randyll's face when Jaime mentions it: he's considering it...and he's accepted :)
So the Tarly men, along with the Lannister army, sack high garden and kill men they've known for years. Dickon even mentions this, that he used to hunt with some of the men he killed.
After Daenerys defeats the combined armies of Cersei's, she offers the men a choice: join her or die. This makes sense. It's a war, and she really doesn't have room or the resources to keep prisoners and they did fight against her (don't come at me saying Dany burned the food, she didn't. Drogon's flames only hit barrels of ale evident in the way they exploded). They knew what they were doing.
Randyll thinks he's being a big tough guy by not kneeling when Drogon roars at the army, and Daenerys addresses him. She gives him three choices: 1, go to the Wall (often a choice for prisoners of war), 2, keep his lands and titles and bend the knee to her, 3, or die.
Randyll blatantly refuses the first 2 options, because Dany isn't "his queen."
So who is, Randyll? The daughter of the lord you swore fealty to and Cersei murdered? Or Cersei herself, who is a kingslaying, queenslaying, kinslaying murderess? Or the daughter of the last Targaryen king, a family you professed such great loyalty to that you defeated the Usurper Robert Baratheon in battle for him?
So, more parallels between the Boltons and Tarlys. They both ended up supporting false monarchs that had no actual claim to the throne despite sitting on it (Joffrey, Tommen, and Cersei).
And what did the Starks do to the houses that betrayed them? Killed them, of course, and the audience went wild with cheers because they're the bestest house everrrr (that was sarcasm).
So Randyll and his son, Dickon, refuse to kneel/swear loyalty to Queen Daenerys and Randyll says she doesn't have the authority to send him to the Wall because she's not his queen, so what's left? Keeping him prisoner? They're in the middle of a war and everyone knows that food stores are tight during a war.
Randyll betrayed his leige's house by swearing to Cersei and assisting in killing the Tyrell matriarch, also stealing from them, and accepting that his house would one day be the new Wardens of the South and Lords of the Reach.
Daenerys had every right to execute the Tarlys for what they did. It was, in actual fact, justice.
Tl;dr: the Boltons and Tarlys were the same scumbags that betrayed their leige lords and got the justice they deserved and it pisses me off when antis use Dany executing the Tarlys as another sign of "madness" when the starks did the same to the Boltons. :)