The international community faces a curious challenge with regards of Sri Lanka. In fact we are in presence of a massive violations of human rights, happened during the end of the war, not to mention also the continuous incidents of unlawful abductions, suppression of press freedom and abuses from the military and organized thugs. The point of the clash is about sovereignty on internal affairs; the argument from Sri Lanka, China and Russia is that any state is absolutely sovereign in disputes within its borders. The way to deal with internal dissent, discontent or even open revolt is entirely national. So not matter how disproportionate can be the reaction against these dissenting forces, it would be always regulated by internal mechanisms.
The West challenges precisely this notion, that any kind of violence is admissible. But it is a opposition largely on principles. In fact it is always too well known that the West screams loudly against violations of rival countries, whilst with allies and friends the protests are definitely quiter.

People’s unrest in Bahrain has been stopped with brutality
courtesy BBC
The example of Lybia and Syria on one side and Saudi Arabia and Bahrain on the other. You don’t need to be a genius of geopolitics to see that the cases are precisely the same and only a decision of convenience makes them different.
You can consider Sri Lanka as a test. Colombo is leaning very clearly towards the non-Western front of China and the World South (Africa, Asia and Latin America), but is not yet a enemy. And this indecision is becoming an accusation against the West. Evidences of violations and war crimes during the end of the conflict are there. Even the UN, even Ban Ki Mon is (almost) tempted to say something. Satellite pictures, mobile footages, Wikileaks reports are so in front of your eyes that it seems almost a joke their denial.

The killing of civilians in Syrian civil war has been firnly condemned by the West and largely publicized.
REUTERS/Shaam News Network
And now we come to the point. Why it is so difficult to make a move and tarnish Sri Lanka with the same accusations? The main justification is about diplomacy: it will cause more harm than help to the victims and to the refugees, such a direct confrontation. It is better to wait. Why? In Syria and Libya you didn’t need any time to wait. With Saudi Arabia and Bahrain the case is already closed. The supect is that you can really decide whether Sri Lanka is an open rival or a distant friend. Only when it will be sorted out, then you’ll know whether Sri Lanka committed the crimes or not. In other terms the violations are against the alignment with the West, not the human rights.
If you can’t decide whether a violation has been committed or not, because you can’t decide on the alliance, then you reinforce the arbitrariness. The defence of human rights is becoming more and more a soft power of the West against China, Russia and other countries of the World South.

China’s vote in the United Nations’s security council
Xinhua
But rapidly this hypocrisy will backfire. The protection of human rights is a principle; if it can be bent to arbitrary necessity, then it is not a principle any more and countries like China are entitled to see these accusations merely as an intrusion.
The case of Sri Lanka reflects even too much this duplicity: the pressure is directly proportional to its distance from the West. The paradox is that if you raise the tone, the country will quickly move far away, so the necessity of escalating the case will make duplicity even more patent. If you remain silent, you’ll gain “points” in other cases, but then you’ll reinforce the negotiable nature of your battle for principles. This is the paradox of Sri Lanka, that prosecution on its violations will uncover the hypocritical fight of the West.
The solution to paradox is of course very simple: you need to stand for human rights always. It is a principle, therefore is not negotiable. The argument is that those principles are better protected by the West, even when they are violated. I don’t know your opinion, but to me this sounds dramatically unconvincing.