Too soon to say categorically suicide/murder
In light of the Delta Incident when the captain was locked out inadvertently and unable to get access back into the cockpit with the code or with the help of the cooperative FO due to a door MALFUNCTION. It is not correct to say that the only explanation for the Captain not being able to gain access is malicious intent on the part of the FO.
The door could have jammed at a time when something was going wrong and the FO was busy trying to handle it. The commanded descent may have been just an attempt to solve a problem we don't know about. The FO fairly inexperienced could have been confronted with something he couldn't handle and froze.