People give too much credit to China on some things. For example, yes, it's the main investor in Sudan, but Sudan doesn't really listen to anyone. China could probably nudge al-Bashir if it really tried, but it doesn't wield any kind of power over the Sudanese government. Further, China hasn't really had a direct role in Darfur. That said, the fact that on the international stage--particularly on the Security Council--it basically represents the interests of non-intervention in the horrors taking place in Sudan (and, for that matter, Burma and Zimbabwe) makes it pretty culpable as an accomplice.
More damnable is its treatment of Tibetans, a group of people who, literally, have almost no cultural or linguistic connection to the Han Chinese, and a tenuous historical one at best. Chinese rule of Tibet is, in many ways, the largest mass-occupation going on in the world today. The thing is, most westerners assume that things in Tibet were wondrous before China invaded. They weren't. It was a feudal, agrarian society, with crushing poverty all-around. But for Chinese apologists to suggest that that is the fault of the current Dalai Lama is not just disingenuous, but a bald-faced lie. Tenzin Gyatso was a teenager when Tibet fell to the Communists. He never actually "ruled" Tibet; his advisers did. He was a kid for most of the time. So, while Tibet's economy has skyrocketed in terms of growth over the last decade or so, its culture, history and people have been systematically repressed and eradicated for 50 years.
America and Europe have over-extended themselves on the issue of Tibet, though. The Chinese have a kind of inferiority complex built into their society. They were pathetically weak and forced to be under the sway of foreign powers for so long that now that they're a large world power, any hint of opposition to its rise from the outside world, and particularly from the "west," infuriates them and unleashes a surge of nationalist sentiment which serves to solidify the Communists' hold on power. Tibet deserves independence, but it's not going to get it just because Americans and Europeans say it should.
I was roundly criticized for linking to the Economist awhile ago, but it has a good
op-ed piece arguing that China's economic rise should be welcomed, but that its human rights record is horrendous, and it did not deserve to hold the Olympics.