Teddy,
In a totally unrelated field I am well versed in confirmation bias, and I would be totally open to any alternative explanation that would at least not contradict what we, at least for now, can take to be facts.
There is certainly no smoking gun and no probable cause, other than to say with reasonable confidence that there is no known failure mode that could explain the altitude and course adjustments made by this flight in conjunction with the lack of any attempted communication or recovery action by the crew. Simply put, if the crew were incapacitated the plane by itself could not have made the turn south, and were the crew not incapacitated, then it is not possible to explain why they did not take other action. What is left is that someone (not necessarily the crew) deliberately caused the plane to take the course it did. Of course this does not give any explanation for motive or finer details on the how, but does give a general picture of what likely happened.
I'll be happy to support anyone who comes up with a theory better fitting the known facts, even if unverifiable beyond the process of elimination.