At least I think that was Martin's point.

We fell for a fantasy trope in thinking her earlier cruelty was justified, even noble. Even if it was justified, it was still sadistic. It was a part of her nature that we shouldn't have overlooked.

They lead us down this line with all the scenes of Ned, Robb and Jon, glumly cutting off the heads that they had to cut off.

I think their point is: nobody is completely noble. Even in a fantasy story, nobody kills just because they have to.

Dany was struggling with inner conflict her whole life, between wanting to be noble, just, selfless; and wanting to be powerful, strong, feared. Wanting to be a human versus wanting to be a dragon.

The coin landed when she finally made it to Westeros, sacrificed hugely to do the noble, human thing, and still everything went to shit.