Rian's intention was to demonstrate some semblance of humanity remained within Kylo. But the optics are that he is truly weak and in the end isn't even bothered much by the (for all he knows at the moment) imminent death of his mother. Had Kylo fired the shot he at least would have surpassed Vader in evilness, whether or not Leia saved herself.
I agree with the "fans" that she should have died in that scene, but since Rian was too scared to snuff Leia before Luke the scene shouldn't have been written in the first place.