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.