So she wants them to see she's different by threatening to kill any that don't follow her? I'm, well, baffled by your interpretation here. I'm absolutely certain that scene was supposed to convey that she was acting Cersei-like, and not that it was some kind act of mercy. It was interspersed with scenes of a queasy looking Tyrion imploring her to do something different, and followed by another seen where Tyrion, clearly trying to convince himself, makes the exact argument you are before concluding with 'I am her hand not her head' which Varys counters with 'Thats what I used to tell myself about her father'. They're hardly being subtle that this is an example of Dany acting like her the Mad King.
And lets also be clear about who she's threatening, the show wanted you to know that the Lannister men are common folk who are tired of war fighting for their pay; they dedicated a whole scene to that very point. They've tried their best to humanise the soldiers. The distinction your making is murky.