North Korea was originally heavily subsidized by the Soviet Union. With their fall in 1989, the state simply did not have the funds to continue their existing operations, and thus fell apart after a year of really bad harvests, which would have come sooner or later.

North Korea is a dictatorship that arose post WWII, as the founder derived legitimacy for leading the anti Japanese resistance. They just continued the traditional top down leadership culture that is part of Asian culture for the past two millennia or so (see China's whole 'Mandate of Heaven' idea. Being a highly collectivist society makes it easier for dictators to flourish, as the citizens are bought up to believe that they must sacrifice personal ideals and happiness for the greater good.

As for the last, it is a matter of geopolitics. North Korea is a buffer state between the US ally South Korea, and China is rather fearful of American influence in the region (they are a strong believer of Thucydides Trap, and believe that in America's position, they would no doubt seek to constrain China as much as possible); thus it supports NK purely in order to counterbalance US power in the region. (Recent Russian support for NK follows the same line, as Russia wants leverage against the US in multiple theaters so it can better negotiate with it on an international level.)