Could you imagine if the cinematic didnt focus on Blue Eyes Sylvanas being tortured, and instead on the Sylvanas we know being tortured by her actions? Not Bluevanas having to sit and watch her Redvanas self be evil, but just Sylvanas being tortured by reliving the reactions to those she killed constantly asking her why she did it?
Just imagine, Sylvanas trying to stop herself from ordering the burning of Teldrassil even as he body moves and talks to the opposite while the captured Night Elves scream in the background. Seeing the desperation of the Forsaken as they try to escape her and be with their families as she has them killed. The seeming confusion in the eyes of Zelling as he is unceremoniously killed for practicing what he was told was the main tenet of Forsaken society.
The idea that she is tortured by the implications of her actions would always be slightly dumb, but at least it could have been more harrowing instead of confusing. After all, Bluevanas might be a true Sylvanas, but from our perspective she might as well be a new character. The scene has about as much weight to it if we transplant any character with a conscience into that scene. You could have had the elf lady that Sylvanas failed to save in the Afterlives cinematic and the scene would have functioned no differently.