Its the directors fault. like, you can script that. But then you have consider how it is told. Music playing. Slow motion, vs fast. etc, etc.
Take the classic example of Caesar being murdered. A moment, that has become legendary. And, yet is a man being betrayed and stabbed. not dying gloriously in battle.
Like, throw some foreshadowing in there. If, his murder is a giant mistake, its as simple as putting stormclouds in the background. There was nothing ominous about the scene. No victory filled with unease. Nothing. no graduate ending. Even that would have worked if that was the plan. They could have given each other the looks as they kill there target. And just hold the camera there. hold it. hold it. At which point the thrill of the kill drops and... ok.. now what.
since none of that happened, what we have is.. nope, he just went out like a bitch. And thats it. Dorne is going to have vindictive queens and that will somehow work out.