North Korean leader Kim Jong Un spent five years trying to kill his older brother before Kim Jong Nam died Tuesday after falling ill at a Malaysian airport. South Korea's spy agency said this week North Korea was behind the alleged murder.

Kim Jong Nam, 46, potentially died from a poison attack at an airport terminal in Kuala Lumpur, Malaysia. Kim Jong Nam told medical workers before his death someone had touched his face from behind and attacked him with a chemical spray. Malaysian police said Wednesday officials had arrested a woman at Kuala Lumpur International Airport carrying Vietnamese travel documents in connection to the death.

South Korea's National Intelligence Service Director Lee Byung-ho told lawmakers Kim Jong Un had long wanted his brother killed, but it took years for his assassins to complete the task, Yonhap news agency reported Wednesday. Kim Jong Nam, who at one time was expected to succeed his father before his death in December 2011 and become North Korea's supreme ruler, had been under the Chinese government’s protection.

Kim Jong Name was the half-brother of North Korea's ruler and had said in media interviews that he had no desire to take power. He was aware his brother was trying to kill him and at one point wrote a letter to Kim Jong Un asking him to call off a standing order for his assassination.

"We have nowhere to go, nowhere to hide. We are well aware that the only way to escape is suicide," Kim Jong Nam said in a letter to Kim Jong Un, one of the lawmakers said.

He fall out of favor in North Korea after he tried to enter Japan in 2001 with a false Dominican Republic passport while traveling with his wife and his son. He explained at the time that the family wanted to visit Tokyo Disneyland. He later moved to China with his family.

"Because I was educated in the West, I was able to enjoy freedom from early age and I still love being free," he said in notes to a Japanese journalist, Yoji Gomi, who wrote a book on Kim Jong Nam in 2012. "The reason I visit Macau so often is because it's the most free and liberal place near China, where my family lives."

Still, Kim Jong Un was worried his brother would unseat him, especially after a 2012 assassination attempt against Kim Jong Nam apparently didn't work out, Reuters reported.

"Kim Jong Un said: 'I just hate him. So get rid of him,'" Kim Byung-kee, a South Korean lawmaker, said.

The two brothers never met under a North Korea tradition that requires potential heirs to be kept seperated.

"I'm his half brother, but I've never met him so I don't know," Kim Jong Nam said in another note to Gomi. "I'm concerned how Jong Un, who merely resembles my grandfather, will be able to satisfy the needs of North Koreans. Kim Jong Un is still just a nominal figure and the members of the power elite will be the ones in actual power. The dynastic succession is a joke to the outside world."

He added: "The Kim Jong Un regime will not last long. Without reforms, North Korea will collapse, and when such changes take place, the regime will collapse."