They shot the plane down on purpose, they took smiling pictures amongst the wreckage, and they prevented the Dutch or anyone else from picking up the bodies.
They shot a plane down on purpose, not the plane. Then they went full CYA mode, as at that time Russian involvement in the conflict was all about barely plausible deniability. Coming out and saying, "yes, we have sent a Russian SAM into Ukraine to shoot down Ukrainian airplanes and accidentally shot down a civilian airliner" was technically an option, but Russia preferred to deny everything and flood the infospace with a hundred conflicting theories.
I just heard this story on NPR a couple weeks ago, it seems that there is unfortunately no conclusive evidence and likely will never be, but they do have strong evidence (intercepted audio recordings) suggesting that Putin was directly involved in supplying the BUK missile system to the separatists that shot down the plane.
