I think it's not just ROK ultranationalists, but a mainstream nationalist mentality that is being taught in schools. During the 2007 Asian Winter Games Changchun, China, young South Korean athletes on the medal podium held up signs that said "Baekdusan is our territory". Naturally this angered China, but the South Korean government only tried to brush it off instead of giving a formal apology.
The South Koreans think that they are the true master of the "Korean Empire". They claim North Korea and parts of China's North Eastern territories. Based the map of ancient Goguryeo. They don't just want all of Changbaishan. They also want Yanbian, chunks of Jilin Province, and possibly even the Liaodong Peninsula.
China was proven right to help North Korea out during the Korean War. If they had allowed the ROK to take all of Korea, an ROK war of aggression would have been imminent. The ROK regime would have claimed Manchuria just like Imperial Japan did. Luckily, the ROK remained as South Korea. They don't have the absolute might to take on the PLA, but their delusions remains. So SK remains a potential adversary in the foreseeable future.