Gemara: [Is it so that there is no death without sin?] Doesn’t it say in a baraita: The angels said before Hashem: "Why did you decree death on Adam?" Hashem answered: "I commanded him in one small mitzva, and he transgressed it." They said to Him: "How about Moshe and Aharon, who fulfilled the whole Torah, and they died?" Hashem answered: "For one outcome will there be for the righteous and the wicked…" (Kohelet 9:2).

Ein Ayah: Even though every person has his own personal moral status based on the choices he has made, there is still a general characteristic of the soul of a human being, which affects a person even in ways that are beyond his ability to affect them by his own choices.
The moral fall of Adam created a set blemish in mankind’s soul. Only when a transformation takes place, one that Hashem, in His goodness, strives for, can he return to a great level of completeness. This elevation needs to occur to the soul of mankind as a whole, and this requires death and all that goes along with it. The individual, even one who has reached a personal level of choices that approaches perfection, cannot perfect his soul by himself. He can only affect his personal status of the soul, not the part of the soul that is connected to the general level of mankind. The latter will occur when all are elevated to the point that "the land will be filled with knowledge of Hashem, just as the waters fill the seabed" (Yeshaya 11:9). Until that point, free choice cannot create human immortality.