Here is an interesting and possible scenario concerning the motivation for firing on the Malaysian aircraft that addresses the question:
Was the downing of the passenger plane a random error of judgment by Russian separatists or a premeditated, deliberate act by Russia?
First, the background facts:
At Putin's request, Putin and Obama arranged to be on the phone to discuss US sanctions against Russia relating to Russia’s incursion into Ukraine. – FACT http://www.ibtimes.com/mh17-crash-obama-putin-discuss-malaysia-airlines-plane-crash-phone-call-over-ukraine-1631764 It can be assumed that Putin established the timing of the call.
During the phone conversation, Putin expressed his displeasure with US sanctions on Russia (the Russian elite). The final words of Putin during that phone conversation informed Obama that a Malaysian passenger plane crashed over Ukraine. - FACT http://eng.kremlin.ru/news/22688
US and Ukraine intelligence reveal that Russian” advisors” fired the missiles that downed the plane. FACT http://www.foxnews.com/world/2014/07/18/smoking-gun-intercepted-calls-point-finger-in-jet-downing-at-russian/
Putin is seeking ways to retaliate against the US for US sanctions against Russia – FACT http://www.thedailybeast.com/articles/2014/07/18/putin-set-to-retaliate-against-obama-by-trashing-iran-deal.html
Putin has expressed his revulsion toward homosexulaity in Russia by saying Russia must cleanse itself of homosexuality – FACT http://www.cbsnews.com/news/putin-russia-must-cleanse-itself-of-homosexuality/
Aids is known to have initially spread through the human population through homosexual behavior and continues to be spread primarily through that behavior - FACT. http://www.lifesitenews.com/news/epidemic-1-2-of-gay-men-will-have-hiv-by-age-50-if-current-rates-continue-w and HERE.
As many as 100 people heading to the AIDS 2014 conference in Melbourne were on the doomed flight - FACT http://www.foxnews.com/health/2014/07/18/aids-community-mourns-as-leading-experts-feared-killed-in-downed-plane/
It is likely that the Ground to Air Missile System used to take down the Malaysian plane was removed back to Russia – FACT http://www.foxnews.com/world/2014/07/19/malaysia-air-crash-missile-launcher-used-to-down-jet-likely-back-in-russia-us/
Based on the above 8 sets of facts, the following represents a reasonable scenario:
The shoot-down by Russian “advisors” in eastern Ukraine was a planned action, not an accident. It served two purposes which could not be passed up:
1) It was timed to impress Obama during Putin's phone call to him. The shoot-down demonstrated to Obama that any further sanctions, or even persistence of existing sanctions by the US on Russia will result in Russia renewing or reinvigorating its support of Iran’s development of nuclear weapons and perhaps actions equal to the downing of the Malaysian aircraft, or worse. This whole episode is a pretext in the context of Russia’s natural predisposition to support Iran against the West.
2) It served Russia’s anti-homosexual agenda as a bonus.
Or was the missile firing at that particular plane at that particular moment merely just an unfortunate accident?