Before the Korean War, there was no concept of a North or South Korea. Although they were under Japanese imperial rule for several decades, they still had an identity as a united people. So how, from 1945 to 1950, did the North Korean government convince their people that they needed to go to war with their brothers and sisters (quite literally) to the South?
The South was portrayed as merely a continuation of the Japanese regime, run by former collaborators. This was, in many respects, accurate. The USAMGIK and RoK preserved much of the machinery of state from Japanese rule, and kept a lot of the same people on. Kim and his pals in the North had solid reputations as anti-Japanese resistance fighters. The war was started to liberate the south from fascist collaborator and imperialist American rule (you can judge for yourself the accuracy of that description or the sincerity of Northerners' motives).
Sources: Cumings, The Korean War: A History, and bits and pieces of Origins.