Right. So do you think the Starks were entitled and brutal when they just didn't let the Boltons to keep the Winterfell? Think all the poor people who died in the Battle of Bastards, think poor Ramsay Bolton who was brutally fed to the dogs (certainly unusually cruel way to kill someone even by the standards of the setting.)
Well, in that case I think if you allowed Winterfell to vote they would have elected the Starks, so that particular case is a bit different.
But I would submit that any other leader on the show with the exceptions of Ramsay and Jaffrey would have accepted the surrender, and at most executed the top generals and the enemy leader. Even Cersei would have brutally tortured Dany if the roles were reversed, but would have stopped burning the city when they surrendered.
There's a huge amount of moral grey in the show, but there's a HUGE difference between killing people in war, and executing people after the war.
So that's why it is in line with her previous actions. No, her prior actions are not objectively worse than other characters' actions previous to that point. But they ARE more predictive of a mass murdering of noncombatants.