I agree that it’s an effective scene in isolation, but as you say it’s more effective for what it promises than for what it delivers. I guess it comes down to whether a shocking moment should remain for its own sake or whether it should serve a more organic whole.

Btw, I didn’t mean to imply that the lightning is intentional on Rey’s part. I would imagine in either case that it comes unbidden to the surface, so I see no conflict with your intent. Rey still has latent darkness that she struggles against and yet is drawn to, it would just be more seductive due to not (apparently) killing her friend.