I've always taken the answer to this to fall somewhere in between, on the one hand, the fact that Óðinn [Odin] is a trickster figure and, on the other hand, the apparent cultural expectation of the Norsemen that the most desirable way to die would be in battle in order to spend one's afterlife with Óðinn in Valhalla, training for the Ragnarök.
Óðinn's trickster aspect appears to me to manifest in Sigmund's youth as well as in his old age. Óðinn has a cameo at these critical points in the life of this great-great-grandson of his. In both instances, before Óðinn shows up, things are going well enough, but the net result of each encounter is basically disaster for Sigmund, although in the first instance it is more indirect.
In Chapter 3 of the Völsunga saga Óðinn chooses a strange way to gift Sigmund with a magic sword at the wedding feast of Sigmund's sister Signy to the Geatish king Siggeir. Óðinn sticks the sword into the Barnstokk, a tree around which the feast hall of Sigmund's and Signy's father Völsung has been built, saying that the person who can pull the sword from the tree will own this weapon, and only Sigmund succeeds at this task. This eventually leads to the death of almost every member of Sigmund's family because he refuses to hand the sword over to his new brother-in-law.
Based on Óðinn's character as a prophet in other stories, it would seem that the god knows all this will happen when he sticks the sword into the tree. (Or, at the very least, none of it takes him by great surprise.) The only reason of which I can think for him to present Sigmund with the gift in this manner is in order to create strife at this wedding feast (whose bride, the same chapter tells us, actively hates her groom from the moment he requests her hand in marriage).
If Óðinn is intentionally sowing discord which will end in the kind of brutality that it does for the Völsungar, perhaps there is redemptive logic therein in that he is essentially a war-god and is seeking among his descendants the most powerful warriors to add to his collection of Einherjar (the dead "Lone Fighters" housed in Valhalla).
Thus, in this way, Óðinn is manipulating the course of his great-great-grandson's life in order to lead him to this final moment in which he will die as a warrior in order to join the ranks of those who shall fight on the side of the gods in the Ragnarök. If that is plausible then it should explain how Sigmund's life ends, and Óðinn's involvement in the scene, which therefore is more of a blessing than a forsaking.
Sigmund's own perspective on the matter is perhaps somewhere in between seeing it that way and simply being resigned about it just being "his time" and "God's will."
The night after the battle, Hjordis went out to the men who had
fallen, came to the spot where King Sigmund was lying, and asked if he
could be made well again.
'Many have recovered when there was little
hope,' he answered. 'But my good luck has turned and so I do not wish
to be made well. Odin does not want me draw sword, for now it lies
broken. I have fought battles while it was his pleasure.'
Chapter 12 - Vǫlsunga Saga: The Saga of the Volsungs, Edited & Translated by
(A potential hole in this hypothesis is that, strictly speaking, Sigmund technically does not die in battle; he merely succumbs to injury.)