Of course, MOTIVE is only the first question you have to ask–Qui bono? Who benefits.
There are LOTS of others which my colleagues–John Moore, Butler Shaffer, and many more– have been asking and analyzing splendidly.
This could obviously be a Black Flag Pack of Lies by the U.S. or a series of stupid mistakes–beginning with the U.S. FORCING WAR on the Russians and Ukrainians, some crazy airline routing directly r a war zone with red hot air war going on, etcetc.
As Aristotle would say, there are many causes. The PRIME CAUSE of this mass plane death is the U.S. war against Russia and the ethic Russian-Ukrainians whose democratic government was overthrown in Kiev with U.S. help and are now freedom fighters wanting only to escape the Kiev tyranny, just like the American Conservative Revolutionaries fighting the tyrannical UK monarch for their British Constitutional freedoms. All the Russians want at this point is autonomy within a Kiev government. The New Russian freedom fighters will probably accept this at this point, as Putin and Merkel, et al, have been quietly, desperately trying to get them to do. Kiev will have to stop sounding like King George demanding the hanging of Jefferson, Adams, Washington and all the American leaders.