The assault was the latest in a series of shooting rampages by bullied soldiers in South Korea, which requires all able-bodied men to conduct about two years of military service in the face of a threat from North Korea.
The soldier, only identified by his surname Yim, told investigators he attacked his colleagues in June last year after seeing drawings they made of him that he considered insulting.
Yim, a conscript soldier, had fled into the forest near the border with North Korea but was captured after a failed suicide attempt.
Today, the high military court rejected Yim's appeal, saying his acts lowered morale among South Korean soldiers, undermined public confidence in the army and caused a "serious vacuum in national security," according to a Defense Ministry official, who requested anonymity citing department rules.
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Yim can appeal to the Supreme Court, a civilian court whose decision would be final, the ministry official said.
South Korean courts occasionally issue death sentences but no one has been executed in the country since December 1997.
In 2005, another South Korean soldier went on similar shooting and grenade rampages at a front-line army unit, killing eight colleagues.
The two Koreas share the world's most heavily fortified border since their war in the 1950s ended with an armistice, not a peace treaty. Occasional shooting sprees at South Korean army barracks have raised questions about the discipline and readiness of its military.