The existence of an arguably worse dictator someplace else with US support does not excuse China from providing support to a terrible dictator in Africa. That logic doesn't work in either direction. For example, if I think the US is doing something bad, I don't feel any better about it if I discover that China is doing the same thing but worse.
Tu quoque is fallacy only if we use it to justify our own actions.
I was not using the argument as a excuse. I was just pointing out that what China does is normal. National interest trumps human rights everywhere. The main difference is that western countries must justify their actions to the public. We use human rights to rationalize wars against enemy dictators while staying inactive towards our own dictators.
Moral arguments from every side in international politics should be seen as propaganda.