South Korea has called the murder an "act of terrorism" and said it was carried out by the North Korean government. At least five North Koreans are currently under investigation by Malaysian police.
But why would Pyongyang want to kill a member of its ruling dynasty? Analysts say it's hard to know with any certainty but differences with ally and neighbor China and the dynamics of sibling rivalry may have played a role.
In 2001, Kim Jong Nam was caught trying to enter Japan
on a forged passport, reportedly in an attempt to visit Tokyo Disneyland.
This caused huge embarrassment for North Korea and ended any lingering chances Kim had of succeeding his father as leader. From around 2003, he lived in near exile in Macau, a Chinese-controlled territory near Hong Kong.
Kim regularly visited China, and maintained close ties with Beijing, primarily through his uncle Jang Song Thaek, the second-most powerful man in North Korea following Kim Jong Il's death in 2011.
"Jang Song Taek was China's guy in Pyongyang," said Jeffrey Lewis, director of the US-based East Asia Nonproliferation Program.
"(He) was the source of Kim Jong Nam's income and probably why the Chinese protected him."
Jang was dramatically purged and executed in 2013
on the order of Kim Jong Un, robbing Kim Jong Nam of his strongest ally in Pyongyang and a major link to Beijing.
If North Korea is confirmed as being behind Kim Jong Nam's death as well, it will "greatly undermine China's confidence" in Kim Jong Un's regime, said Zhao Tong, an associate at the Carnegie Tsinghua Center for Global Policy in Beijing.
"Kim Jong Nam long advocated for a pro-reform approach in North Korea and openly encouraged (Pyongyang) to follow China's example," said Zhao.
China is North Korea's only real ally, but relations have become increasingly strained
as Pyongyang has continued to aggressively pursue its nuclear program in the face of international sanctions supported by Beijing.
"When the North Koreans executed Jang, it was nominally for his business dealings in China
," said Lewis.
"North Korea is wiping out all the pro-Chinese regime elements -- although this murder seems especially cruel."
Such a move would be a dramatic miscalculation by Pyongyang, according to Zhao.
"If this murder is confirmed (as being ordered by Kim Jong Un), that will deal a major blow to China's hopes about the North Korean leadership's ability to open up," he said.
This could fundamentally change how Beijing -- a long advocate of diplomatic talks between Pyongyang and its rivals -- deals with North Korea and its nuclear program.
A recent decision by China -- citing UN sanctions -- to halt all coal imports from North Korea
, may be a sign of Beijing's displeasure with Kim Jong Nam's death, Zhao said.
Sidelined after his father left his mother for dancer Ko Yong Hui in the 1970s, Kim Jong Nam was at one point a potential rival to his youngest brother for the succession (a middle brother, Kim Jong Chul was passed over for unclear reasons).
Nevertheless, Kim Jong Un's ascension progressed far more smoothly than many predicted
, and he soon shored up his grip on power through a brutal campaign of crackdowns and executions.
A South Korean think tank said in December that Kim had ordered the killing of 340 people since 2011
Kim Jong Nam lacked anything close to a power base in Pyongyang, according to Michael Madden, an expert on the country's leadership.
"Given his heritage, (Kim) Jong Nam was viewed by some elderly North Korean elites as a kind of grandson figure," he wrote last week
. "This affection and relationship could not necessarily form a basis of political support domestically, but it would have been helpful had (Kim) Jong Nam ever put himself forward as a political rival to his half-brother."
However, it is unclear whether Kim Jong Nam ever desired to succeed his father, let alone his brother.
In interviews with Yoji Gomi, author of the 2012 book about him "My Father, Kim Jong Il, and Me," Kim criticized hereditary succession and called for economic and political reform in the country.
Speaking to former UN Under-Secretary General Elisabeth Rehn in an interview for Finnish television
, Kim Jong Nam's son Kim Han Sol said his father was "not really interested in politics."
Any potential plot to challenge Kim Jong Un would have likely required support from China, something Zhao doubts would be have been forthcoming since 2011.
"It makes no sense for China to engage in political conspiracy against Kim Jong Un and risk the overall China-North Korea relationship," when the chance of Kim Jong Nam succeeding "is so low," he said.
Born while Kim Jong Nam was studying overseas, and brought up separately by a mother who saw her husband's first family as rivals to her own sons, Kim Jong Un was never going to be close to his eldest brother.
Indeed, according to the author Gomi, the two brothers never even met.
This did not stop Kim Jong Nam openly criticizing his sibling, saying that Kim Jong Un was too weak to maintain control over the country and was a puppet for regime elders.
Such criticism is unlikely to have gone down well in Pyongyang, which often reacts angrily
to any perceived slight, particularly from overseas.
Speaking to reporters last week, Gomi said Kim Jong Nam's comments were known within the country.
"A defector told me there was a rumor the oldest son of Kim Jong Il had said critical words about North Korea and could be a cause of (reform)," he said. "That person mentioned the rumor gave him hope."
South Korean lawmaker Lee Cheol-woo, citing a National Intelligence Service briefing, said that Pyongyang had been attempting to assassinate Kim Jong Nam for five years.
A North Korean man jailed for spying in South Korean in 2012 reportedly said he had been ordered to kill Kim.
Another lawmaker, Kim Byung-kee, said that Kim Jong Nam had written to his brother in 2012, asking him to spare his life and those of his family.
According to the South China Morning Post
, friends of Kim Jong Nam in Macau said he told them he felt he was living on "borrowed time."
Zhao said it was possible the timing of Kim's death -- awkward for North Korea as it faces massive international condemnation for its nuclear activity -- may have been a coincidence.
"Kim Jong Un himself may have agreed for the North Korean spy agency to track down his half-brother, but he may not have directly issued the order to kill him, especially at this moment," he said.
The opportunity for the alleged assassins may simply have been too good to pass up.