The probably were on their way to Somalia.
My guess: I'm assuming the news reports that this plane flew for 4-5 hours are true. So I think it was simply hijacked and was heading for a landing field that was beyond the range of the aircraft. As a result they simply ran out of fuel and crashed into the Indian Ocean. The hijackers were too stupid to know that their target landing place was out of range.
Probably terrorist but not necessarily. Could be simply an aircraft kidnapping for ransom.
Draw a straight from where it was last seen to where it is found and project that line to the target landing field and you will know who the bad guys are.
And no, I don't get paid for this sort of excellent analysis. I just give it away for free. ;)