First it's ridiculous to condemn the current leadership for crimes of the past. Second, China's support for North Korea is simply politics which the US engages on a daily basis. May I remind you that the US was a great supporter of Saddam Hussein when it was convenient to do so because he was opposing Iran. The US was also furnishing the same Taliban that they are now fighting with weapons when it was convenient since they opposed the Russians.
Yet, I wouldn't say it's Obama's fault that the US invaded Iraq. It seems he's somewhat trying to extricate himself from these delicate positions just as the chinese government is trying to move forward from the mistakes that were made in the past.
In terms of world politics, I'd say the US is a greater problem than China but at the end of the day, both countries are out to protect their interest, it just happens that the US has its fingers in more pies than China.
However, I'd still prefer to live in the US than in China. Neither are anywhere near the top of my list but there's more work to do in China to make it appealing as a place of residence for its citizens.