Ok - there is some new information that would explain a sound motiv behind the Co Pilots actions:
A search of the said First Officers appartment revealed a medical doctors statement, that he is medically unfit to go to work and that he would be so for a longer period of time. He decided to keep this to himself and not inform his employer, as it would be evident, that they would suspend him from flight duties indefinately. This confrontation was devastating for him, being an enthusiastic Aviator from a very early age and he felt his life ruined. Instead of staying home, as the doctor prescribed, he decided to fatefully go flying in this state of mind...