Yep, having them killed wasn't out of line. But she had to have them burned and tortured. That's an act of a tyrant. She was losing grip on her claim at that point (they were losing the war up until that battle) and reverted to tyrannical measures to assure she doesn't lose grip. I don't think Dany is setting out to be a tyrant but is turning into one. When shit hits the fan she reverts to tyrannical behavior. She will do anything to get her claim on the throne.