The argument that Randyll Tarly "deserved to die because he was an ass" is a strange one
If he was an ass, his final scene wasn't evidence of it. By James Faulkner's performance, by the lines that he was asked to speak, and by his principled objection to refuse the offer of the Black on the basis that he did not acknowledge Dany's legitimacy as Queen, we were clearly being invited to admire him, if not
necessarily to agree with his stand. Hell, he tried to save his son's life.
The point being that it is wholly reasonable to be opposed to Dany's rule, because she is a stranger to Westeros, and because the forces backing her consist of father-murdering advisers, treacherous spymasters, slaughtering bandits, and monstrous beasts.
Now as to whether Dany was right to execute him and his son, on a pragmatic basis, I'd say that's arguable. Beheading is a lot less likely to evoke memories of her insane father than burning alive through dragonfire, though.