If you really have a pilot that is focused on a suicide/mass murder plot, a two person flight deck rule doesn't seem like much of a hurdle. If you are planning to kill everyone anyway, why not just start off by bludgeoning/strangling/stabbing/slip a ruffie/etc. the pilot next to you and then the two person rule is bypassed. If the pilot looks to be too hard to handle, maybe the replacement FA would be easier.
I don't think you understand the possible thought processes of people in these tragic situations. If it was suicide, but wasn't terrorism related or to make a point, he may have simply done it on a whim. The opportunity presented itself and he just went along with it to see what would happen. Just having another person sitting there could easily have been enough to prevent the tragedy, even if he thought he could over-power them in a fight.
It's too early to say, though. The point is, you're applying a rational thought processes to a situation isn't rational.