You could be right, but even in the world of psycho serial killers, this would be extremely out of the ordinary. Serial killers generally want to get their hands dirty, not sit in what amounts to a separate room while all his victims are elsewhere. They didn't even know they were in danger until the final seconds.
He could be crazy, but I still suspect a political motive and Islam is the dominant political movement that uses these tactics. We'll learn more in the coming days.
Think we need to keep in mind that pilot credentials can be stolen; see the link below. If no one knew this copilot on sight, perhaps he was killed and his stolen ID used by a flight trained terrorist. One of the passengers worked for a company doing satellite intelligence.