"Surviving alone is too painful while 200 remain unaccounted for. I take full responsibility. I pushed ahead with the school trip," his note said.
"I will once again become a teacher in the afterlife for my students whose bodies have not been discovered."
After the police cut down his body, he was taken across the road to the Jindo funeral parlour and cremated. His ashes, according to his wishes, will be scattered on the ocean where the Sewol sank.
The news of his death caused barely a ripple inside the Jindo gymnasium, where Mr Kang had stayed with hundreds of parents, waiting for news from the rescue operation.
As hope has faded, the gymnasium has become a cauldron of anger and despair. On Friday, medical staff darted around the hall, attending to parents convulsing and screaming in grief. At least 20 parents have had treatment for shock at the local hospital, a doctor said.
View: Crew criticised over Ferry disaster
Relatives were quick to blame Mr Kang and other teachers for the fate of their children. "I saw him on Thursday afternoon around 4pm," said Kwon Hyeok-ryung, 55, whose brother-in-law was also a teacher at the school, but is still missing.
"He felt a lot of responsibility, and he was under a lot of pressure. The parents blamed him for surviving. They screamed: 'How can you be a teacher and let your students die? How can you live with yourself?' They were grabbing him, trying to beat him."
Around an hour after Mr Kwon saw Mr Kang, he walked out of the gymnasium and disappeared. A police search began during the night.
Meanwhile, prosecutors were busy on Friday night filing arrest warrants for Captain Lee Jun-seok, 69, the third mate who was steering the ship at the time of the disaster, and another crew member. The three men are likely to face criminal charges of deserting their ship after being among the first to leave the boat.
Kang Hye-sung, 31, the crew member who made the fatal announcements telling passengers to remain where they were, said he had been following orders from Captain Lee. "It was so hectic in the ship that I couldn't even think to make any judgment," he said.
At the wharf in Jindo, hundreds more parents and relatives continued to hurl abuse at South Korean officials as the hunt for survivors became an operation to retrieve corpses.
"You have stopped the rescue, now you just want to pull out dead bodies!" one father screamed at a police liaison officer.
"I have not slept because we parents have had to do everything ourselves," said Kim Hoonmin, 42, whose daughter is still missing. "Last night I went out to the site until 2.40am. We hired a robot submarine to try to shoot video of the inside of the boat so divers could analyse it," he said.
"And we wanted the robot to clear a path for divers. But why do we have to do these things? Why aren't the officials doing them?
Roughly 200 civilian divers, many of them military veterans, have arrived at the wharf to volunteer to help the operation.
Some teams managed to retrieve bodies trapped in the external areas of the ship but were forbidden by the navy from going inside the ship in intensely treacherous conditions.
"The visibility is down to seven inches," said Kim Suho, 42, the chief of one civilian dive team. "But that is not the problem. The problem is the currents are swirling inside the ship so that it is impossible to open any doors. And there is a vortex of turbulence under the ship at a depth of 110ft which sucks divers down."
The civilian and navy divers have succeeded in securing guide ropes around the ship to help them manoeuvre in the darkness and silt within. On Friday night, Korean media reported that divers had reached the third floor of the ferry, where the ship's cafeteria lies, and the second floor car port and container hold.
High pressure hoses were also piped into the ship to pump in oxygen, in the faint hope that it might reach survivors trapped in air pockets.
A giant crane arrived at the site to haul the ship out of the water, but the salvage operation will not begin until the parents on the wharf and in the gymnasium have consented and could take months. "The crane has nothing to do with the rescue," said Mr Kim. "It is just to save the ship, not to save the students. If they lift it, anyone who is alive might die."
At the wharf, lines of ambulances gathered waiting to ferry victims to local hospitals and a makeshift morgue was assembled. As night fell, a huge floodlight was installed to allow parents to inspect the bodies that were expected to emerge before morning.
Progress was so slow, however, that only three bodies were brought back to the wharf during the day's operations. As they were stretchered out, a huddle of parents peered over them to try to identify them, before a woman screamed and buckled. "They found her, they found her," she sobbed, clutching two others for support and standing in the middle of the road in blank horror.
Ferry captain arrested
Meanwhile, a prosecutor says the captain of the South Korean ferry that sank two days ago has been arrested.
Senior prosecutor Yang Jung-jin says 68-year-old Lee Joon-seok and the crew members were detained early Saturday.
The ship, the Sewol, sank Wednesday, leaving hundreds missing and feared dead.
The investigation into the disaster has focused on the ferry's sharp turn before it started listing, and on whether a quicker evacuation order by the captain could have saved lives. Investigators also were determining whether the captain had abandoned the ship.
Rescuers are searching for about 270 people still missing and feared dead.
At least 28 bodies have been recovered. Officials said there were 179 survivors and about 270 people remain missing, many of them high school students.
Where to get help
* Youth services: (06) 3555-906
* Youthline: 0800 376-633
* Kidsline: 0800 543-754 (4pm to 6pm weekdays)
* Whatsup: 0800 942-8787 (noon to midnight)
* Depression helpline: 0800 111-757 (24-hour service)
* Rainbow Youth: (09) 376-4155
- with additional reporting from AP
