Killing off Kim Il Sung would help, but it would depend on who ends up in charge. The most likely candidates are Park Hon-yong, who would spend more time and effort trying to encourage a communist uprising in the South; Mu Chong, who would probably be a more sophisticated and pro-Chinese version of Kim Il Song; or Ho Ka-i, who would have created a smaller, more Soviet and technocratic regime in the north. Neutralizing Rhee in the south would help as well. Maybe if the North didn't invade, he would fall from power earlier.
A better PoD would be having Cho Man-sik accepting a four-power trusteeship period in which the USSR, US, UK and China were meant to set up an independent government. This would be a much more complicated arrangement than the division in OTL. The USSR thought that they could use Cho, but his intransigence led to them opting to support Kim instead. A messy scenario wherein there are Commonwealth and Chinese occupation zones on the Korean peninsula may provide scope for Cho Man-sik and other nationalists (even, maybe, the domestic communists like Park Hon-yong) to put together a plausible central government and opposing people like Rhee, Ho, Kim and Mu as foreign agents of one stripe or another. This might prove enough of a headache for both sides to agree to a nuetral, democratic Korea with both camps thinking they can fenangle it into their own lines later on.