well, if the pilot was intent on suicide and intent on taking another 137 passengers with him then he could probably easily have killed/disabled the co pilot at the point he diverted the plane
Or vice versa - co-pilot could have done the same.
Which, to be honest, from the families point of view, is probably better than knowing for sure that your loved one was responsible. Better to never know the truth, if that is what the truth is...
I don't know, just find the whole thing so very upsetting for everyone involved.