It is extremely rare for any one particular group to do it, but it DOES happen. It is extremely unlike that anyone will commit murder, but it still happens frequently around the world.
Something happened to this plane. And so far, the theory that best fits the known facts is that one of the pilots did this intentionally.
It could make sense if one of the pilots, distraught or facing a crisis of some sort, wanted to lash out, and end his own life. He flies the plane to the hand off point, where its disappearance wont be evident for some time, disables the transponders, eliminates the passengers, and turns around. The flight path from there does not appear random, but designed to evade detection and then off into some of the most remote ocean on Earth. Not leaving a note is not uncommon in suicide. And I think it was part of his plan, for this to be a mystery.