I'm going to try to give you the answer I think you're looking for, because it's the answer I've been looking for too- the reasoning Russia uses behind it's criticism of the investigation into the destruction of flight MH17. Generally they have arguments which are fully fleshed-out, although nevertheless transparent, for the benefit of their people.
I'll start with the previous coverage of the incident. I think it's important because Russian statements, as said above, are primarily for their own people. Manipulation of the media in Russia means that they can get the people to support whatever action the government wants to take. The official statements are for them, to reinforce what they're being told on the news.
I've found that the Wikipedia article on MH17 has a lot of useful information. For instance, it says Russian media, in the aftermath of the incident, blamed the Ukrainian government.
The Russian government-funded outlet RT initially said that the plane may have been shot down by Ukraine in a failed attempt to assassinate Vladimir Putin, in a plot which was organised by Ukraine's "Western backers". Other theories propagated by Russian media include: that the Ukrainians shot down the plane in a botched attempt at mass murder of Russian citizens or by mistake (reported twice, in July and in December); that Ukrainian air traffic controllers purposefully redirected the flight to fly over the war zone; and that the Ukrainian government organised the attack on the plane to bring infamy upon the pro-Russian rebels.
Given everything they're hearing, it must be easy to imagine that Ukraine was responsible and is covering it up, backed by the West which wants to blame Russia.
It's worth noting that there have been those who have railed against this: for example,
Sara Firth, a correspondent with RT, for which she had worked over the previous five years, resigned in protest at the channel's coverage which she described as "shockingly obvious misinformation". RT issued a statement after Firth went public with reasons for her resignation, saying "we were not surprised by Sara Firth's decision to leave RT after five years as a Moscow and London correspondent, as she has recently informed us that she was likely to take an offer from another firm".
The investigation has been led by the Netherlands, who lost the most from the crash. Most have been supportive, with U.N. political affairs chief Jeffrey Feltman saying:
The United Nations is confident that the Dutch-led international investigation has been conducted in accordance with Annex 13 to the Convention on International Civil Aviation Aircraft Accident and Incident Investigation
However, Russia has been critical of it the whole way through. Vitali Churkin, the Russian Permanent Ambassador to the UN, said the following:
As for criminal prosecution, it is carried out by members of the GIT in a closed fashion. It was said that an agreement was reached between five sides for not disseminating information, in this case what are the grounds to be assured of the impartiality of such an investigation? Can this investigation resist the aggressive propaganda backdrop in the media? Can it resist pressure of clear political origin, when the causes of the disaster and those that are guilty of this disaster are named in advance, when such statements are made by a number of leaders of certain states which form part of the GIT.
His reasoning, which may be valid, is that many have already made up their minds about Russia's culpability. Additionally, because they're keeping a lot of information to themselves, it's difficult to ensure impartiality. On top of all that there is a clear political interest in blaming the pro-Russian rebels and, ultimately, Russia.
There's another element to this as well: the appropriateness of the UNSC's involvement with a criminal investigation. The UNSC is chartered to ensure the security of the UN. Russia contend that the resolution, creating a tribunal to prosecute the perpetrators, is outside of that purview.
The Kremlin has also issued a statement saying:
The Russian president confirmed the unchanging position that it is inexpedient to create such a judicial body
So apparently there are timing issues too, possibly because the investigation hasn't finished, so they're creating a tribunal to prosecute before there's anyone to prosecute. I think they're afraid that action will be taken against them regardless of what happens with the investigation, which may be a genuine concern, although not unjustly.
Beyond anything else, if the whole world claims Russia provided the weapon, their own people could start to listen. Putin, and the entire Russian government, is dependent on their people not listening to what the West says about them, only paying attention to their own media. If Russia is found to be at least partially responsible, some will realise they've been lied to, which is enough doubt to cause significant issues. So prosecution like this could cause problems internally, and they'd lose any remaining international credibility.
Reuters article on Russian criticism of the investigation
PressTV article on the veto
Churkin's statement to the UNSC
Al Jazeera article on the veto