After roasting a significant chunk of Jaime Lannister's armies and all of his loot train, Daenerys Targaryen rounded up the survivors and demanded they either bend the knee or be sentenced to death. Tyrion Lannister tried his best to reason with her but she chose to make an example out of Lord Tarly, Samwell's father, and Dickon, heir to House Tarly. She sicced Dracarys on the father and son, burning them alive. It was a harsh move that has fans wondering: is Daenerys the Mad Queen on Game of Thrones? It does look like she's following in her father's footsteps.
It seemed unnecessary to Tyrion, her hand, to kill soldiers who had already surrendered. First, he begged Dickon to reconsider his firm decision to stand alongside his father and die, rather than continue the Tarly name under a Targaryen rule. (Lord Tarly was obviously far too "honorable" to betray Queen Cersei, even though he didn't even serve her until literally five minutes ago when his queen, Olenna of Highgarden, died by Cersei's command.) Then, Tyrion gently suggested to Daenerys that she imprison Dickon until he comes to his senses. But Dany didn't want to set a precedent for leniency, even though she's supposedly a merciful queen.
The thing is, her bar for mercy has always been kind of all over the place. She maintains that she allows her followers to choose her as their ruler, rather than force them into obedience. But she also watched her husband murder her brother, murdered a bunch of masters in Meereen herself, and burned a temple full of Khals alive. If someone holds any kind of power at all and is considered a "bad guy" by Daenerys' standards, then she has no mercy at all.
No one was more unsettled by her brutal call on the battlefield than Varys, who served at Mad King Aerys' side, watching him burn enemy after enemy. He confided in Tyrion after the attack that he feared Daenerys was behaving suspiciously like her father. The imp is obviously beginning to feel complicit in Daenerys' brutality, and Varys validated his concerns. The Spider pointed out that it became harder and harder to tell himself that the Mad King's actions weren't his fault as he stood by and watched the kingdom burn. (Sound suspiciously like any non-fantasy political climates you know of?) Daenerys seems like she's just barely able to discern between who's a friend and who's a foe at this point. And it's possible her gauge will completely break before the season is over.