I believe that the paladin should uphold the agreement made (he passivly agreed by not raising any objections). That's part of being Lawful - you should stick to your word.
The paladin never agreed. If you're being arrested by a cop, are you passively agreeing to his assumption of guilt because you go along with him (with little/no protest)?
Also, a lot of people on this board seem to feel that being Lawful means never or not being able to lie or break an agreement. That's not true either. Lawful people everyday, everywhere tell lies in some magnitude. Maybe to spare someone hurt feelings, maybe something more dramatic, but it happens. And it doesn't make them Chaotic to do so. Or even Neutral. It just means they are Lawful and lie on occassion. Remember, part of being Lawful (Good) is taking the concerns and welfare of society over your own personal needs (such as the compulsion to fulfill your word). No one in their right mind would think the paladin wrong for not continuing his agreement with the imp. Besides his class prohibits him from continuing such agreements without facing the consequences of losing his status as a paladin.
When it becomes obvious that it was a trick and the terms of the agreement have been satisfied he should Smite the imp at the earliest oportunity.
Which he did! And, after the terms of the agreement was met. Yet some people around here still think the agreement was in effect when he took out the imp--and it wasn't!!
I can't accept that he was decieving them by remaining invisible. He was withholding info, but they knew that info had been withheld. If there was something that info could be that would make the deal unacceptable, they should have insisted on clarification before agreeing. But any deception was essentially agreed to by the party.
That's about the dumbest argument I've heard on this topic so far! Whatever the reason (player inexperience, in-game time constraints, etc.), that proposal is even more ridiculous than everyone else believing the paladin even agreed to the agreement to begin with! The fact is that the imp was deceiving them by remaining invisible. Not a single person in that group would have agreed to work together with the inp had it revealed its nature ahead of time. Hell, the paladin probably would have Smitten it on the spot! Again, I'm sure there were plenty of reasons for the group not being able to sit down and ask the imp (or anyone else in any other encounter) 20 questions until they were 100% satisfied on the creature's identity, intentions, goals and motivations.
If the Imp had been disguised as something else and make the deal in that guise, that would have been deception. But when you make a deal with something that you clearly and explicitly don't know the identity of, you don't get to cancel the deal later because you didn't know their identity.
Says who? Invisibility is an even bigger deception than disguise. And sure you do. Why? Because the imp deceived them! Agreements, like contracts, are made in good faith on both sides. If one side breaks that faith (such as by withholding information that would result in the contract never having been made to begin with--or information that would prevent one party from being bound to the terms of the agreement), such as the paladin learning the identity of the imp, then the agreement is not valid. The paladin would actually be forbidden from continuing an agreement with the imp as that would have directly been furthering the cause of evil. And for a paladin to further the cause of evil is an evil act which means the paladin now becomes a feat-less Fighter.
If you are offered a low cost cruise on the basis that "you will share your room with someone whose identity will be reveal upon check-in, to be referred to as "Cruiser X" in the remainder of this document" and you agree to it, you have not been deceived if you check in and it's your old college ex. You can't say "I wouldn't have agreed if I knew I'd be sharing a room with HIM!" because you knew you could be sharing a room with anyone, and if there was a limit to that anyone that would have made you not accept the deal, you should have tried to clarify it before signing.
In your example, nothing is prohibiting said cruiser from having to stay in that room, change rooms with someone else or even stay on the ship and not get off at the next port. That example has next to nothing to do with the OP's post.
Anonymity is not the same thing as a false identity. One attempts to decieve, while the other lays the ambiguity out to be accepted or not.
It is not the same when the ambiguous side knows that the other party won't agree to a damn thing if it revealed itself. That's called a deception, or lie of omission. All you're trying to do with your argument is blame the victim (oddly enough, not the one that was killed). What you're arguing is no different than saying someone who got raped or beaten up deserved it just because they couldn't fight off their attacker.