If you're family with the history between these countries at all you'd realize how erroneous it is to say that North Korea should have any more reason to hate Russia, China, or Japan over the United states. There's definitely some deep rooted historical backing the the fact that the older generations, and some of new as well, in Korea and China hate the Japanese. Some of the atrocities committed by Japan in China and Korea were pretty well on par with the Holocaust.
The United States definitely holds that gluttonous, gun toting, Manifest Destiny reputation in world terms but the thing is we don't have any real backing to it anymore. The Cold War started with us being the first to get our hands on Nuclear weapons and then trying to strong-arm essentially the whole world so Russia felt the need to counteract. When Russia created the SDI to counteract ICBM's the USA then created a missile with even more warheads than the SDI could stop - the thing is that an SDI costed far more than an ICBM so it was detrimental in budgeting. That's basically when the conclusion was had some resolution should be made.
The Korean War was really just a vessel for Cold War tensions with Russia and China backing the North and the States supporting the South. Russia helped train North Korean troops and even supplied them with ammunition/artillery. They pushed so far down into South Korea before the States decided to send help but then when we regained North Korea's capital China sent over some 500,000 men to push us back down to the 38th Parallel.
I can't say I necessarily believe North Korea in all their claims but just think they like nitpicking with the United States. They know China has their back and we're in the palm of China's hand so we're not going to do shit....but then neither are they. I think they just get a kick out of saying these things, watching the states get their panties in a bunch, then laugh when they try to rawl up some BS U.N. Sanction to no avail.