>>All that Build-Up of Avenging “Da Renlys”, I can’t see her finding it in her heart to let him go.<<
I thought that too, but then thought more on his line - “Go on. Do your duty.” Might have struck her that her duty to protect Sansa is greater than her desire for vengeance. Perhaps it changed her mind, and she just struck the tree above his head (another off-screen “death” cliffhanger). She’s not a murderer. I can see her bringing a wounded Stannis to Winterfell as an exchange for Sansa or something along those lines.
It was interesting how they showed Stannis’ death off camera. Most of the time, they show deaths on camera (Ned Stark, the Red Wedding, Joffrey, etc.). On the other hand, there was Syrio Forel.
Maybe you’re right and they’re not quite done with Stannis yet. However, he did have a pretty bad leg injury, and Brienne isn’t a healer. I guess we will have to wait and see.