I will do this, and I see it as a highly legitimate tactic.
If I can't win any longer, why not try to take an enemy down with me? Because they 'expect' me to fight everyone to the bitter end? Because doing X, Y, or Z is in my 'best interest' for a year or two, until they screw me again?
No, if I no longer have a reasonable chance of survival in a game, my goal *sometimes* shifts from "win, draw, or at least have a few SCs left" to "take the bastards down with me".
Sure, I could drag the process out, and possibly survive a few more years, but I find it much more satisfying to watch the (metaphorical) look in the eyes of the stabber as his or her carefully thought out strategy collapses in the face of my 'irrational' moves.
IMO, there is no 'illegitimate' strategy in Diplomacy (outside of outright cheating), including the way you choose to die. If I ever play a game with said stabber again, they're likely to at least *hesitate* before replicating their stab - and hence, my 'irrational' move give me an advantage in future games.
As for Pandarsenic's point of acting as a Janissary, this again falls into the 'do as I say and you might live a little longer' bucket. Sometimes I'll do it hoping to salvage a 'Survived', sometimes I'll do it hoping for a stab later - and sometimes I'll say 'screw it' and go down swinging. If the individual who expects me to be his armor doesn't like my choice, well, frankly, tough luck - assuming that someone will act in the way you want/expect them to is foolhardy in the extreme in this game.