The court of appeals found that the man was not guilty of murder, instead ruling that he had committed manslaughter.
"It cannot be ruled out that this person, who has a disability, hadn't acted in the heat of the moment," judge Margareta Bergström told the TT news agency. "We don't know what happened, or why it happened."
The sentence was shortened to ten years in jail, down from the 14 year-murder sentence handed down by the district court in January.
Ten years is the maximum penalty for manslaughter in the Swedish penal code.
The woman disappeared in northern Sweden last year, sparking a massive search and rescue operation. Parts of her body were later found in an abandoned building in Boden.
The case took an even more morbid turn when it was revealed out that the woman's killer had joined the volunteers scouring the countryside for her body.
The ensuing police investigation, however, turned up neither a murder weapon nor a credible reason for her being killed.
On Wednesday, the court of appeals said several factors had been taken into account before the sentence was downgraded. Chief among them, the failure to pinpoint a motive. Details were so hazy, Bergström added, that the court did not know why the man had taken the woman's life.
The court also took into account the man's "peculiar personality". He suffers from Aspergers and ADD.