You're really trying to twist my explanations around to suit your own needs, aren't you? :\
And you seem to be completely ignoring the nature of fiends in D&D. We're not talking about ordinary, evil humans who deceived the party. We're talking about damned souls that have gone to the Lower Planes and been twisted into even-more-Evil creatures that exist only to perpetuate more evil still. They are formed pretty much from pure Evil. Only the rare fluke, like A'kin (JUST MAYBE; we still don't know if he's really just unspeakably evil and puts on an act, or if he's actually a 'risen' fiend), and I mean extremely-rare fluke, has some abnormal inkling of Good in them that wasn't purged when they became fiends.
Yet even those ones are composed of more raw Evil than any mortal human. The absolute worst human is still suffused with less raw Evil than the fiends who are created from Evil itself, even the 0.00001% of them who somehow retained a tiny shred of Good during their creation (and you have to keep in mind that most of those ones get destroyed by other fiends for their 'flaw', whereas fallen celestials are more likely to just be banished or something; fiends are never so merciful to those among their own kind who possess the 'taint' of Good).
THE POINT BEING........that Imp has already committed a lifetime of sins when it was a mortal, and continued to commit sins after its 'rebirth' as a fiend on the Lower Planes. It has already murdered, corrupted, abused, violated, poisoned, betrayed, or orchestrated the doom of several other mortals before it ever met the party. YOU CANNOT JUST IGNORE THAT. It is not a fiend just because it happens to do evil things sometimes. It is a FIEND because it has ALREADY GONE TO HELL for its atrocious sins. It is already a damned soul. It is already MORE THAN WORTHY OF SMITING.
You cannot simply disregard the reasons for why it is an Imp in the first place, or what terrible evil it has already perpetrated. It cannot even be redeemed unless it is one of the rare, flawed fiends, whom other fiends are likely to have already destroyed anyway. If it were one of the virtually-unheard-of few 'risen' fiends that escaped Hell intact, it would be trying a lot harder to figure out its purpose and how to either redeem itself or purge the 'taint' of Good within it.
It would not be deceiving a party of adventurers, whom it almost certainly had more than enough time to scan for Good while it was invisible before they had any idea it was even there. It would have either been direct with them and tried to get their help honestly, to prove it wasn't going to use the item for evil and that it was trying to redeem itself, OR it would have avoided them altogether and left, or waited until after they had gone in and then tried to retrieve the item afterwards, invisibly.
It would not have followed the course of action that it did, in the manner that it did, unless it was as thoroughly, irredeemably evil as fiends generally are, or was giving in to its evil side and forsaking redemption rather if it was in fact 'tainted' with a shred of Goodness.
That Imp already deserved to be destroyed by the paladin, whether or not it had caused any direct harm to the party of adventurers yet.
It was already a creature of terrible sinfulness and villainy, not just a corrupt mortal, and was already one of the Damned. The paladin needed no direct, personal reason to smite it, for it had already horribly wronged, probably killed, other mortals beforehand. It was already an incarnation of pure, cosmic Evil, that would only commit further evil as long as it lived. And there is no doubt that it had already committed terrible sins against fellow mortals beforehand; while possible that it had not killed anyone yet, it is not likely, and the imp still committed enough sins beforehand to go to the Nine Hells of Ba'ator upon its death as a mortal.