I agree. The fact that this could even happen is a big, big problem. And there is no way any pilot could have done this on his own.
A pilot could easily lock the co-pilot out if the co-pilot goes to the head. If he then dumps the cabin pressure, it would be virtually impossible to mount an attack on the cockpit, and the pilot could simulate severe turbulence to incapacitate anyone not strapped in.
Like it or not, pilots have a lot of ways to murder everyone on the plane without any assistance. If the pilot wants to commit suicide, he could follow the above steps then take off his own mask. Nobody would be conscious and the plane would cruise until it runs out of fuel.