'Like a spy movie': In Seoul, assassination of Kim Jong Un’s half brother engrosses, doesn't amaze Kim Jong Nam was poisoned to death by two Asian women in Malaysia.

 -- The assassination of Kim Jong Nam is presumed to have stemmed from North Korean leader Kim Jong Un’s “paranoia” about his half-brother, according to South Korea’s top spy agency. Kim Jong Nam was poisoned to death by two Asian women at Malaysia’s Kuala Lumpur International Airport on Tuesday.

Public reactions in Seoul are of shock and horror, if not amazement.

“We all know how brutal and psychotic Kim Jong Un is, but news every hour has been more like a spy movie,” said Park Choon-ho, a 60-year-old taxi driver.

“Kim Jong Un is cruel and cold-blooded, but this [assassination] only shows how insecure his political power stands,” said Kim Do-yeon, a college student.

Getting rid of the eldest son of the late Kim Jong Il has long been a “standing order” from Pyongyang, said Lee Byung-ho, the chief of South Korea’s National Intelligence Service.

North Korean agents attempted to assassinate Kim Jong Nam in early 2012, which led him to write a letter to his younger half brother, asking for mercy for him and his family.

“I ask for you to cancel the punishment order on me and my family. We have nowhere to go and nowhere to hide. We clearly know that the only way to run away is to commit suicide,” the letter read.

Kim Jong Nam's first legitimate wife and eldest son, Kim Han Sol, reside in Beijing. His second wife and their son and daughter have been living in Macau. The family members are under protection by the Chinese government, the National Intelligence Service confirmed.

Seoul has doubled the number of security guards protecting North Korean defectors who held senior political positions in Pyongyang.

The South Korean military announced plans to spread the news to North Koreans by resuming broadcasts on dozens of loudspeakers facing the North along the countries’ shared border. North Korea keeps almost all outside information from its residents.

Surveillance footage from the domestic check-in area at the airport showed two young Asian women approaching Kim Jong Nam. One woman was wearing a white T-shirt emblazoned with the block letters “LOL.” Kim then complained to a ground attendant that someone had covered his nose and mouth with a piece of cloth and that his eyes burned. He died in an ambulance on the way to a hospital. Police in Malaysia have arrested an alleged secret agent, a 29-year-old wmoan traveling on Vietnamese documents.

An autopsy of Kim's body has been completed at a morgue in the presence of the North Korean ambassador to Malaysia, Kang Chol, but the results are still unknown.

ABC News' Hong Yoo, from Seoul, and Maureen Jeyasooriar, from Kuala Lumpur, contributed to this report.