Actively suppressing democracy is not doing a bad job of supporting democracy, it is actively suppressing democracy.

The south was not a sovereign country, they were being ruled by a US installed dictator who was killing many of his own people (look up the Jeju Uprising for one example). I'm not talking about living in North Korea today, I'm talking about the Korean War and the history of US aggression and the influences that has. North Korea was objectively more free and prosperous than South Korea until the fall of the Soviet Union. You have to remember that after Syngman Rhee was overthrown South Korea was ruled by a military junta until the 80s.

The facts are that the USSR and the Korean people (many of them being led by Kim Il Sung) were the ones who drove Japan out of Korea. The US didn't want the USSR to have influence in all of Korea so they drew a line at the 38th parallel. The USSR agreed to that line because they didn't see it as a hill they wanted to die on (they didn't want to chance a war with the US since they had just lost 27 million people in WWII).

Is that a reference to how much napalm the US used on the Korean people? If so that's pretty sick to joke about genocide like that.

If the US was trying to support democracy then why did they destroy the people's councils that had been established in the south, fill the government and police force with former Japanese collaborators, and install a dictator in Syngman Rhee? And then after Rhee was eventually overthrown, why did the US support the military juntas that came into power?

You said it yourself when you said China didn't let North Korea get conquered. The US wanted to conquer all of Korea, and the "first punch" was when the US arbitrarily divided the country, and conquered the south.

After Japan was kicked out of Korea people in both the north and south started developing their own democratic systems. Then the US arbitrarily divided the country at the 38th parallel, and established a military occupation in the south. This occupation destroyed the democratic institutions that had been set up, and kept officials and collaborators from the Japanese colonial government in power. Then the US installed a dictator to rule the south. Syng-man Rhee made it his mission to suppress and kill anyone he thought might be socialist. Given that context I don't think it was that North Korea was trying to force anything on the south, but that they were trying to drive an occupying force out of their country, like they had done with the Japanese.

The US already threw the first punch with the Korean War. They were also the first ones to bring nuclear weapons to the Korean Peninsula, and they have been openly calling for the destruction of North Korea for years while ignoring any attempt for peace.