Vision’s death was just one of the major gut punches in a film that ended with a barrage of them and depending on your perspective, it may not even be the saddest (Peter Parker’s death is rough), but it did have the best build.
It is made all the more tragic when Thanos reverses it, only for Vision to be killed again and discarded by the Mad Titan, making the lovers’ sacrifice all for naught.
Vision asking something no one should ever have to ask of a loved one and her having to commit this act of mercy and salvation is incredibly agonizing and played beautifully in the film.