Your mistake was to fail to tell the other players about your plan and agree, particularly with the paladin player, that this was going to be okay. The paladin player's mistake was not to ask you what you were doing, before attacking your character. You are both at fault, as players, because you aren't co-operating with each other. You aren't playing as a team.
The DM is at fault for letting the attack take place. Having said that PvP was not allowed, he should not have allowed it.
You must stop secretly operating against your fellow players. Talk to the paladin player, tell him what the DM has said and arrange with him how you want to proceed as a team. You could make a running joke out of it, if you make one purposely bungled attempt after the other and the paladin always somehow avoids getting caught in increasingly bizarre and improbable ways.
Paladin, at breakfast: "I woke up suddenly in the night and found a varlet poisoning my waterbottle. Before I slew him, he confessed and claimed he was hired by a masked assassin. I claim 10XP."
You: "That's a remarkable coincidence. During the night, as I was prowling around the inn, I overheard a masked assassin offering a servant 10gp to poison someone's drink. I slew the assassin and took his purse of gold. I claim 30XP and 25gp."
DM: "Hold on guys, what's going on here?"
You and the paladin player, keeping a straight face: "You tell us."