Discussion about what or how someone is made an ally or enemy without actually utilizing the discussion? The saboteur question is easily figured out. 'When you have a game rule, and no exception exists to the game rule, apply the damn rule.' Discussions of motivations, plot, narrative, all that... those are irrelevant to the rule element at hand.
An ally is any creature, other than yourself, who is a willing recipient of your powers. Everyone else, other than yourself, is an enemy. Nothing more. Nothing less.
In the case of the saboteur:
While he is infiltrating, ask if he is a willing recipient of the PCs' powers. If yes, then he is an ally for the purpose of those powers. If no, then he is an enemy.
Generally, the saboteur may decide to be willing. That makes him an ally.
Now let's say you have Alice, Bob, and Chuck who are the PCs. Darlene is the saboteur, a tinker gnome with wierd gadgets and what not.
Darlene is a willing recipient of all their powers. That makes her an ally for their powers.
Alice and Bob don't suspect anything. They decide that Darlene's alchemical tricks and artifice are pretty cool and would benefit them. They are willing to receive his powers. That makes them allies to Darlene.
Chuck doesn't trust Darlene. He decides not to be a willing recipient to her powers. That makes him an enemy to Darlene.
This creates the situation where Chuck is Darlene's enemy, but Darlene is Chuck's ally... and yet Darlene is the one who's supposed to backstab!
How is this possible?
Because 'ally' and 'enemy' when refered to in game elements do not mean the same thing as ally and enemy in a narrative sense. Trying to attribute narrative definitions to what are in actuality strictly game terms is just as incorrect as saying that all members of the monk class must belong to a monastery. It's a game term, and nothing more.
If the example of Darlene being an ally, but Chuck being an enemy confuses you, that is because you are not applying the game rule, and that is why you have trouble with players thinking you can change your allyness to prevent friends from being pushed through your square... they are literally taking advantage of your misunderstanding of how it works.
Townsfolk you just met? Probably doesn't trust your powers. Enemy. Saboteur who plans to stab you in your sleep? Ally.
It has nothing to do with their inevitable narrative intentions or whether or not you even like them. It only has to do with one single question: 'Are you a willing recipient of their powers.' If yes, ally. Otherwise, enemy. No other consideration matters, and such a determination can be easily made simply by the actions of the characters involved. Narrative can provide an answer to the question the rule demands, but narrative alone does not -define- ally or enemy.
The above is why the defiler has to have 'willing or unwilling ally' as who he defiles from. Because he IS using a power on other PCs, they could normally evade it if they decided they were unwilling, becoming enemies by dint of no longer being allies. By making allies able to be unwilling allies for that one power, it means that all that needs be concerned is if the ally is generally willing to be a recipient of the defiler's powers.
Of course, there exists a strong possibility that a group with a defiler might have members that are enemies for the defiler's powers to avoid being defiled. Such individuals trade the risk inherent in the defiler's enemy-blasting magics hitting them for safety from the defiler's ability to... well defile. This doesn't mean that they are enemies in the narrative and therefore the defiler and them must fight to the death. It just shows where they stand for the defiler's powers.
tl;dr: Narrative is irrelevant, if the saboteur is willing, he's the pc's ally, if the pc is unwilling, he's the saboteur's enemy, and at no point does this require attacks or blood be spilled to define.
