The worst type of terrorism would be to make an aeroplane vanish, then do it again.
Pilot suicide is an easy way out for the Authorities.
Slow decompression ala Helios has to be a strong possibility perhaps with one last confused pilot input or passenger input befiore she went down. Smouldering fire may have the same effect and take out systems also.
The turning off of systems has not been adequately explained, black box may do so.