It seems unlikely that both Captain and First Officer would conspire to bring about the vanishing act. Have the Malasian police turned up any evidence of collusion? We know the Captain was concerned about recent political events; he may not have intended a fatal outcome.
The first officer was handling the RT, and all seemed routine up to the incomplete handover. If the captain was acting alone, what excuse could he make to the first officer for turning off course? a technical failure that required a return to base? a call from the cabin crew that somebody was setting fire to the waste bin in the WC and the First Officer was needed to sort things out? This could leave the captain alone locked in the cockpit to carry out his plan, which may not have included vanishing at sea. Apoxia leads to inappropriate decisions.