A South Korean soldier went on a deadly shooting rampage, as he felt humiliated and insulted by his comrade-in-arms who drew mock cartoons of him as SpongeBob SquarePants, military investigators have said.

In June, the 22-year-old sergeant identified only by his surname, Yim, shot dead five fellow soldiers at a guard-post near the North Korean border and wounded another seven before fleeing into a forest.

He was captured after a botched suicide attempt at the end of a two-day manhunt.

Announcing the results of a three-week investigation into the shooting, military authorities said that the sergeant was bullied at school and continued to be the target of mock and derision also after he joined the army.

The last straw that tipped him over the edge was seeing drawings fellow officers made of him on the back of his unit's border patrol log, where he was depicted as a series of goofy cartoon characters, including SpongeBob SquarePants from the famous US animated series.

"Our investigation found that he reminisced about the stress he suffered due to being ignored and mocked by senior officials and fellow soldiers after joining the Army," said the head of the Army's Military Police, Brig. Gen. Seon Jong-chul.

Seon said that soldiers saw the drawings "as a small joke" but triggered in Yim flashbacks of when he was taunted at school and caused him great distress.

"Yim thought he could not live well even after being discharged from the Army. He thus thought of killing all of his fellow soldiers and then killing himself," Seon explained.

Army officials also disclosed the content of the sergeant's suicide letter.

"In my situation, anybody might have felt that it was much more painful to live than to die. I am to blame for much, but they are also to blame for some," the note read, The Korea Herald reported.

"Children may harass dogs, may kill insects or worms without any guilty feelings. But they don't really understand how much pain their actions bring."

All able-bodied South Korean men are required to serve in the army as the country face constant threats from North Korea. Yim was enlisted in 2012 and was only three months away from being discharged.

Defence Ministry officials said military prosecutors were planning to indict Yim. Six members of his unit who allegedly bullied him are facing disciplinary punishment.