Killing kindess or a courteous-sword
Here is an item, a variation of which I just recently added to my own campaign (a gift to a half-orc fighter by a crazed dwarf he had help save). I stole the idea from a Scarred Lands campaign I was in. I think this is along the lines you were thinking.
The group cleric identified the weapon as a +1 sword. The item is intelligent and only communicates telepathically. In battle, before the fighter takes his turn, the sword sends a message to the fighter's mind (what little there is) that says something to the affect of "You look nice today." or "That was a fine blow." So far, the fighter is fairly certain that the messages are coming from the axe and he has asked his companions for help, but none of them have offerred any suggestions (some have actually agreed with the voice's assessment of the fighter's fighting prowess.)
What the fighter does not yet know is that the sword is actually trying to entice the fighter into activating the command to unleash the greater power of the sword. When the sword receives a complement, it acts as a +2 weapon (if it receives an amazing complement-as determined by the DM-, it acts as a +3 weapon).
The origins of the weapon is that it was the product of a gentleman knight who had the blade crafted in this manner before giving it as a gift to his noble squire. Thus, the knight ensured that his squire always recalled the codes of chivalry, courtesy and honor (especially when he was killing things I suppose
).
The fighter is currently at 4th level. If he has not figured out how to activate the +2 by 5th level, a wandering bard may inform him of the weapon's history and by 6th, he will probably be told.
In the campaign I stole the idea from, the weapon was an axe wielded by a barbarian, but the barbarian already knew of its abilities. (Still there were times in the heat of battle where the barbarian forgot to complement the axe and so never got the +2 benefit). The barbarian was 5th level so perhaps it was a little overpowered, but it was so much fun for the DM and the players (plus the badguys were pretty overpowered in that game too, the nature of the Scarred Lands I suppose).