There are three answers.
1) what do you mean by “we”?
2) what do you mean by “support”
3) what do you mean by “abuse human rights”?
Broadly, I think the question you’re trying to ask is “why do democratic, liberal Western governments maintain cordial relations with governments that do a variety of seriously bad things beyond those typical of a liberal Western government?”, correct?
You give the examples of Saudi Arabia and China. While I think those were just meant to be examples, they can both serve as examples.
Saudi Arabia is a large, populous, rich, militaristic, and stable country that makes up the bulk of the Arabic peninsula. It borders most countries in the region. Destabilising it would probably have consequences for its neighbours, as well as for supplies of oil and other, less-heralded natural resources. Its large geographic area would make governing it hard, its military strength would make fighting it harder than fighting the Iraqi army, its large population means any war would hurt a lot of people. Those same things also make it a useful ally, as it was in the Gulf War, and it would be if the West wanted to intervene in a neighbouring country. We would rather have it as our ally than scare it away to ally with Russia and/or China against us. Finally, it is a useful counterweight to Iran, which is somehow even worse, and has allied strongly with Russia.
China has the largest population and largest economy in the world, as well as the second strongest military and the third largest nuclear arsenal. We cannot reasonably overthrow the regime. However, in recent years the West has pivoted away from viewing China almost like India (getting richer, getting freer, a useful if unreliable partner) to viewing it more like Cold War Soviet Union (a potential existential threat which should be disengaged from as far as possible). China is now seen more as an adversary than as an ally. That said, it is too rich and too powerful to ignore completely.
Most human rights abusing countries are not as dramatic as those two. So let’s think about Eritrea. It has very little strategic value, and there is very little reason for other governments to fear it. It could hardly become less stable. So why don’t we intervene in Eritrea? For the very same reasons: it holds little strategic value and it is not a threat. Why would we send soldiers to die there? Sometimes humanitarian arguments will work (and they are personally arguments I would use!) but most governments aren’t going to listen to them unless they really get through to the public. After the Iraq War, there is very little appetite in the West for further military adventurism.