The captured sergeant apologised to his family and the families of the five soldiers who were killed in a shooting rampage last Saturday in a note he wrote right before he attempted suicide on Monday, the Defence Ministry said.
The 22-year-old sergeant, only identified by his surname Lim, underwent surgery for a self-inflicted gunshot wound between his chest and left shoulder, and was recovering at a hospital in Gangneung, Gangwon Province.
Medical staff injected him with sedatives and put him to sleep to prevent contingencies.
"In a note, ... Sgt. Lim apologised to his own family and the bereaved families," said ministry spokesperson Kim Min-seok during a press briefing.
"He repented, saying that what he committed was huge."
Lim was captured alive on Monday after special commandos and his father and older brother persuaded him to surrender.
Last Saturday, he detonated a grenade and fired some 10 shots in and outside his barracks, killing five soldiers and wounding seven others in his unit of the Army's 22nd Division near the eastern inter-Korean border.
Medical team and military officials wait outside the hospital ward in Gangreung, Gangwon Province, on Monday. (Yonhap)
Local media reported that the note also indicated that bullying might have been a possible cause of his shooting rampage. He was reported to be on bad terms with his senior and junior colleagues, who treated him as an outcast.
Lim's family also told media that they saw signs that he struggled to get along with his fellow soldiers.
"When he came out on a break last month, he talked little and looked frail, a reason why we were worried (about his life in the military).
(We guessed) he was on bad terms with his colleagues," a family representative said.
During his school life, Lim did not get along well with his classmates, according to his family.
His troubled relationship with them was a reason why he dropped out of school and opted to take the state exam for a high-school diploma, his parents said.
The funeral ceremony for the five victims is to take place on Friday, the ministry stated. The ceremony will be presided over by the two-star division commander.
The ministry also said that the five victims will be honoured as martyrs of duty, not as war dead.
The bereaved families have demanded that the victims be designated as war dead, a status highly respected in the country.
The investigation team was gathering statements Tuesday from wounded soldiers and witnesses to determine the exact motives behind the shooting.
After Lim recovers, he will be questioned, officials said.
Under the military criminal code, a soldier who kills his or her senior staff or sentinels is to be given capital punishment or life imprisonment.
But given that the country has not carried out the death penalty since 1998, a life sentence is likely for Lim, legal pundits commented.
Meanwhile, the military authorities came under fire for using a fake soldier to pose as Lim to divert the attention of reporters thronging the hospital.
Most media outlets carried photos of Lim being transported to the hospital, but the man lying in a stretcher with a blanket covering his body turned out to be a different one.
The military said the medical staff requested that a fake soldier be prepared, as throngs of reporters could hamper the process of treating Lim.