How you interpret is up to you. But there was no 'weaseling' or any of that other nonsense on the part of the paladin. They agreed to work together to achieve mutual goals. They did. Agreement met and ended. There was no agreement for safe passage or anything else and the DM, knowing that it was an imp making the deal could easily have had the imp (in the bargaining process with the group) state, "Ok, we work together. I help you, you help me. We get our items and part as friends. Good?" But no, there was nothing mentioned about that. And that's not loopholing. The agreement was done. Its terms were met and thus it ended. Upon meeting the terms of the agreement, the imp could have hauled butt out of there at least until the paladin was gone, but for whatever reason decided to stick around.And I consider both of those weaseling, loopholing, and lawyerin' unworthy of a paladin.
And you're full of it if you think that silence does equal compliance. Just as an easy, immediate example; if that were the case, everyone arrested or cited would be instantly found guilty of whatever they were being charged with and there wouldn't be a need for a judge or jury (or legal counsel for that matter). Think about that next time a cop pulls you over, serves you with a warrant or just flat-out arrests you.
Reserving judgment, by remaining silent on a matter, is not being sneaky or deceptive or misleading in any way. If the paladin was the only one not vocal about his agreement, then it should have been obvious to the rest of the group there was some doubt or hesitation on his part, at which point, they should have checked with him on it instead of just assuming he agreed with them because he didn't say anything. But they didn't. They just assumed. But that's still irrelevant anyway because the agreement was fulfilled in good faith by both sides.
Whether the paladin meant to or not, he did not break the terms of the agreement. The terms of the agreement/implied truce was met and fulfilled. Period. If there is any doubt of that, go back to the OP and re-read it. The paladin did not kill the imp until the agreement/implied truce had ended. He isn't anywhere in the wrong at all on this.