How do you know it was accidental? That hasn't been confirmed and the converse that it was a deliberate act hasn't been confirmed either. The routing of MH17 was well known in advance and it would have been familiar route to many people. That shoot down was a shoot and scoot mission and if they were after Ukrainian Air Force aircraft why wasn't the launcher repositioned elsewhere within the rebel controlled area for further action because they still had 3 missiles left.
Was it a shoot and scoot? Or was it a normal air defense mission followed by an "oh shit we got an airliner" run across the border?
There's several things to consider. First off they initially claimed, the rebels, that they shot down a Ukrainian military transport. A silly claim to make if you
knew you were shooting at an airliner. It basically admits guilt. Second off, Ukrainian aircraft had been in the area that day, despite their current denials, and there were eye witnesses. Third off there were air resupplies flown into Lugansk earlier where Ukrainian troops were surrounded. Now Ukrainian troops were surrounded at Izvarino, and it's not impossible that they thought similar attempts might be made, hence the deployment of the Buk systems there. Fourthly, this has been discussed before, but the Buk TELAR by itself is incomplete. It's meant to have a btln command point, and a search and track radar, not just its own FCS radar. Multiply an incomplete system by an incompetent crew and mistakes become increasingly likely. A final thought. What would the purpose be? Clearly it brought Russia nothing but trouble. What possible motivation could there have been? Russia has agents within Ukraine, including the military. I would be surprised if Russia didn't have the ability to actually use a Ukrainian system to do something like this instead of the whole circus with a Russian system entering and then leaving. Not to mention that no special efforts were made to hide its entry. It entered, like other Russian military aid, fairly openly. To the point where internet bloggers were able to map its path long before the investigation report came out.
There's also a couple of things that clearly point to a political motive behind many of the accusations. Keep in mind motivation doesn't invalidate the facts but it does mean we have to consider the implications more carefully. First off Ukraine was treated as a full partner in the investigative proceedings despite being far less then reliable. Doctored materials and staged photos are a regular feature of Ukrainian officialdom. It got so bad that some of their evidence submitted to the US, of Russian involvement in the war, was doctored. Why is this so bad? Because plenty of real evidence was to be had. But they were so used to manufacturing it that they never bothered. Meanwhile Russia was basically presumed guilty and Russian materials were treated accordingly. Second off there were demands for a war crimes tribunal before any conclusion had been reached. Certainly long before such nuance as intent could be examined with any degree of impartiality. And yet an accusation of war crimes
requires intent. This to me says that the conclusion was reached by political decision makers for political reasons long before real evidence was examined, and the process of examining the evidence was merely procedure, with no real impact on the political demands and pressure that Russia has and will face.