Here are several major problems that will be a major hassle for the United States and its allies:
* Supposing that you have killed c.500 million people in China. The 750 million survivors are going to demand humanitarian aid and medical relief. Refugees will flood into Hong Kong, Japan, Taiwan, Singapore, Malaysia, Thailand, Indonesia, India, Nepal, Vietnam, North Korea, and the Philippines. With the exceptions of Hong Kong, Singapore, Taiwan, and North Korea, all of these nations have had a long history of anti-Chinese sentiment. You are going to have anti-Chinese riots, refugee camps, ethnic cleansing and border conflicts...
* Second, with the situation in Russia and China, if people think the problem of "loose nukes" was a problem in 1991, imagine the problem magnified with the decapitation of the governments. In this scenario, it is very easy to see rogue states (Iraq, Iran, Syria, North Korea, Indonesia, Mynanmar, Cambodia, et al.) and terrorist organizations (e.g. Japanese Aum Shirinkyo, Russian Mafiya, Chechen guerillas, Abu-Sayyef, Taliban guerillas, Al-Qaeda, et al.) all obtaining nuclear weapons...
* Third, with North Korea, with the sudden influx of Chinese refugees, and the sudden capture of Chinese nuclear weapons, they can either sell to terrorist organizations, or can use nuclear blackmail against South Korea and Japan. In this case, Kim Jong Il and Kim Il Sung won't have the Chinese and Russian governments pressuring them to the diplomatic table....
* Fourth, any computer IT person can read and detect a computer virus, and can often trace where it came from. Just remember that Chinese "Brain Drain" means that many of the computer programmers of China, studied abroad at places like Stanford University, Cal Tech, et al. As such, within 48 hours, they will know that it was the Americans who caused this genocidal act, plus the evidence to prove it before an international audience
If even one of these scenarios takes place, the United States and its allies are going to be in a world of hurt. More likely, it will be a combination of two or more items, with the possibility of a mushroom cloud appearing Washington D.C. within 2-5 years almost certain....