There were reports out there that Kim Jong-nam was running short on cash in Macau so a US intelligence agency trying to take advantage of this seems possible. It also seems very possible that this would give Kim Jong-nam the excuse he needed to take out his brother as well:
KIM Jong Nam’s alleged ties with a US intelligence agency contact may have been the reason behind his spy novel-like assassination, Malaysian authorities said.
According to Japanese daily The Asahi Shimbun, Jong Nam, the estranged half-brother of North Korean leader Kim Jong Un, was carrying US$120,000 when he was killed in February at the Kuala Lumpur International Airport 2 (klia2) and is believed to have received the money from the contact.
Malaysian investigators said Jong Nam had met with an American man who is believed to have links with a US intelligence agency during his eight-day trip in the Southeast Asian country. This, they said, may have prompted North Korea to silence Jong Nam.
Quoting unnamed sources, The Asahi Shimbun report said Jong Nam received the money during his time in Malaysia and had not planned to declare the cash when leaving the country. The authorities also said there were no records of Jong Nam making any large cash withdrawals from Malaysia.
A check of Jong Nam’s belongings, however, revealed wads of cash – all in US currency and in four bundles of US$100 bills – in his black carry-on bag, [Asian Correspondent]
You can read more at the link.