The sword turns out imperfect because of the interference in the middle of crafting the sword, which makes Grid super mad.
But because the materials were so expensive he finishes the sword anyway even if he knows it won't be as good as it could have been.
The sword basically becomes a cursed sword that absorbs Grids hatred and other negative feelings.
The funny part about the sword is that it deals bonus damage to Grid because it hates Grid for "abandoning it" while it was being forged.
So he basically makes a legendary cursed blade who wants nothing more than to kill Grid slowly and painfully.
Thus giving it some really good bleed procs and other neet effects.