His case became a milestone of Ukrainian justice as the war raged on: Sgt. Vadim Shishimarin, a 21-year-old Russian soldier, was sentenced in May to life in prison after pleading guilty to shooting a 62-year-old riding a bicycle in northeastern Ukraine.
On Friday, the Kyiv Court of Appeals reduced his sentence to 15 years, saying it would provide the reasoning for its decision on Aug. 3. His lawyers had argued that he had not intended to kill the victim, Oleksandar Shelipov, when he shot him in the northern region of Sumy in the early days of the war.
Ukraine’s justice system has come under criticism in the past from human rights advocates for imposing life sentences in which the only possibility for release was terminal illness or a presidential pardon.
During his trial, Sergeant Shishimarin accepted responsibility for following the order to shoot the man, who had been talking on his cellphone; he said the phone raised fears that Mr. Shelipov might report the presence of the sergeant and the small group of other Russian soldiers he was with.
“I was ordered to shoot, I fired an automatic burst at him, he fell. We drove on,” Sergeant Shyshimarin told the Ukrainian Intelligence Services. During the trial, he acknowledged that he was not bound to follow an illegal order.
The Russian subsequently apologized to Mr. Shelipov’s widow, Katerina Shelipova, after she gave emotional testimony, asking him: “Did you come to defend us? From whom? Did you come to defend me from my husband that you killed?”
Sergeant Shishimarin told her, “I understand that you will not be able to forgive me, but I apologize.”
The case was the first successful conviction of a Russian soldier accused of a war crime, and the outcome was a seminal moment in Ukraine’s attempts to prosecute Russian soldiers for wartime atrocities.
In announcing the original sentence, Judge Serhiy Ahafonov pronounced Sergeant Shishimarin guilty of violating the laws and customs of war and of committing premeditated murder.
Sergeant Shishimarin was part of a 40-mile long convoy of armored vehicles snaking from the Russian border toward Kyiv, Ukraine’s capital, which Moscow initially expected it could take within days.
According to prosecutors, Sergeant Shishimarin was commanding a tank division from the Moscow region. When his convoy came under attack by Ukrainian forces on Feb. 28, the Russians dispersed. Sergeant Shishimarin met four other men, who stole a car and tried to drive away.
From the car, in the village of Chupahivka, they spied Mr. Shelipov, who was talking on the phone as he rode his bicycle. Believing that Mr. Shelipov would report their location to Ukrainian forces nearby, another soldier — who was not Sergeant Shishimarin’s superior — told him to shoot, prosecutors said.
Experts said the trial was one of the swiftest in Ukraine’s recent history.