At the beginning of the Assassin's Sword quest, you receive a +13 Att, +1 Initiative sword, which can be equipped immediately.

At the end of the quest, regardless of whether or not you complete it, stop it, or destroy the sword, you are left with a +13 Att, +1 Initiative sword.

Clearly,

1) the sword should be significantly more powerful at the end of the quest if you complete it, given that you've gone and slaughtered some innocents (and, presumably, Kingdoms won't like you much anymore)

2) the sword should not exist anymore if you've destroyed it.