It was in motion as soon as Gendry was set free. The showrunners skillfully had us see Stannis develop a relationship with his daughter, but kept us in the dark that he planned to sacrifice her all along.

Are you sure you read to the end?

I think we're meant to look at all of that in retrospect, and realize he wasn't softening as a character but planning this all along. As in he may have got caught off-guard by a few moments of closeness with his daughter but that it didn't deter his singleminded pursuit of power. There was no nice Stannis, just the…

Rorry Rorge.