Pontius
I think that "everything" is on the list of probable cause at the moment, however abhorrent some of them may be.
The Malaysian authorities have used the term "manual intervention" in respect to the deactivation of the transponder. That is what they have stated.
If they are correct (and "we" do not know), the reason for that alleged "intervention" is unknown. As is the person or persons responsible, their technical knowledge and background(s) or their motivation.