For me assuming that a player in a role playing game actually wants to role play and not abuse the system seems a pretty healthy assumption. I would rather assume good faith on the part of the players. Presuming bad faith is the entire premise behind alignment mechanics as you yourself say.
So, again, presume good faith on the part of the players (and no similar presumption for the GM), the mantra I have come to associated with [MENTION=22779]Hussar[/MENTION]. And yet...
But, at this point, presuming we had an honourable LG 4e paladin, doing this would be very out of character would it not?
Why are you presuming bad faith on the part of this hypothetical player? Can he not have concluded that, in the interests of Good, this compromise to his principles must be made? That it is, in fact, more honourable in serving the greater good - the benefit of all those this villain will go on to harm (in the same manner he has harmed so many already) by sacrificing his own principles than it would be to place his own prestige as "an honourable warrior" in priority to the best interests of all those people? Seriously, does the Paladin consider it more important that he look good in the history books, or that the people be freed from this tyrant?
So why is he doing it? If the player cares so little about playing in character in the first place, alignment isn't going to fix the problem.
In my example, I suggest the Paladin is playing perfectly in character. You seem to presuming that no one sincerely playing their character in character could possibly play that character differently than how you envision him being played. I don't agree that the Paladin cannot deviate from the precepts of LG and still be played "in character". What this scene establishes, in a game where alignment is used, is that this character's viewpoint may not perfectly coincide with the ideals of "LG", and that he will follow his principles, even if that means risk of loss of his status, his powers, etc. That seems to me to carry the potential for a stronger game than one where:
(a) The player says "Code of honour says no, so no poison".
(b) Hussar says "Poison? What happened to your Code of Honour"? so the player abandons that idea, because other wise he will be perceived to have violated your perception of how he should play his character.
(c) The player says "Using this poison is consistent with the ideals of Honour" and thus it is deemed that his use of poison is consistent with the ideals of Honour, and great songs and epic tales are woven of Perry the Poisonous Paladin and his unshakeable honour in poisoning the tyrant king.
You can sub "LG" for "Code of honour" in any of the above and it's not a great result either, frankly. But it doesn't matter whether it's mechanical alignment or some other presumption of the need for the player to stay "in character" as you view staying in character.
And how do we know that 4e Paladin is LG, much less what the precepts of LG are, if we are not using alignment. I don't think the Paladin [MENTION=42582]pemerton[/MENTION] refers to in his 4e game is LG anyway, is he?
Again we're back to policing the players. If we don't hold that stick over them then of course the will ignore their character. I mean it was said so pretty clearly a few posts above this. Alignment is apparently required to make players play in character.
Why not trust that the players will actually be capable of playing in character?
So, then, it is OK for the NG character to slash the throat of the fellow tied to the chair, and he is still virtuous and holy, perhaps even more so, after taking that action. And the Paladin may choose to use that poison without compromise to his honour - or even enhancing his honour. Is that correct? Or is there some "stick"* that you would apply in your game should such an event occur? That you interpret any possible "out of alignment" action suggested as "an insincere player playing out of character" suggests to me that you do not, in fact, have the trust you suggest we apply above.
Which actually makes more sense - "players are sincere and GM's are not" is a tough one for me to wrap my head around. "I am sincere but I have little confidence in the rest of you" seems much more consistent, at least.
* Funny...any penalty related to alignment is a "stick", but any other negative consequence to a character arising from the rules of the game are just part of the game, and natural consequences of the characters interacting with the world. In larger than life fantasy, where Good and Evil are not just abstract concepts, but actual forces of the universe, it seems to me that consequences of interacting with the world can easily include consequences related to those real forces.