Someone determined to crash a plane would not have too much of a problem incapacitating any cabin crew member (by surprise most likely) before his attempt. I believe that having a second person in the cockpit is more useful (and seemingly essential) if the Pilot/Co-Pilot loses conciousness in order for the door to still be opened.
Killing someone is extremely easy if the other person trust you. You get behind her and hit her with a heavy object in the head or in the nape.
If the pilot is busy doing something like piloting the plane, it is very easy for a copilot to get rid of her.
That makes it physically easy, but the psychological demands of that situation are clearly very different from turning a knob. I think there's good reason to think that there are people who would do the former, but not the latter. If the pilot is inclined to crash the plane due to overwhelming depression, for example, I doubt that this pilot is likely to also kill a flight attendant with his bare hands.
If true, the only case where having a second person present would be one where an unrelated emergency develops that requires immediate attention at the moment the first person becomes unconscious. I think that scenario is handled by requiring both officers to be in the cockpit at the times where such emergencies are most likely to develop (take off and landing)
Also, that second person would have to be able to fly the airplane. That would mean having a 3-person crew flying the plane (something that Airbus has worked hard at getting rid of for cost reasons), or having a bathroom in the cockpit (impractical, I guess)
In this case I think a remote operator could have seen that something was wrong for minutes, and could have taken over control of the plane, but of course, having that ability carries its own risks.
In this specific case, it may be possible that the co-pilot would not have forced the situation where he was in sole control of the plane.
The emotional side of me wants to call the man a coward, and a coward wouldn't be openly aggressive towards a flight attendant, but that's not a very productive point of view. In future cases, there's just no reason why a future murderous co-pilot couldn't be more aggressive, so it's still a situation worth considering.