Bwahahahhaha...

Probably Itachi killed Kakashi's friend Obito. And Kakashi took out his eye for himself. Probably there is something to it, probably Obito asked Kakashi to take his eye. If not Kakashi wouldn't just took his eye for no reason.

However at one point, Kakashi was probably a brooding hero just like Sasuke, so your theory has a point, that Kakashi would go as far as to take the town's corpses eye just for himself.. to reenact revenge on somebody.

It's most proabable that Kakashi taken his friends Obito's eye. But it's still the question of who killed Obito. Maybe the murderer is dead already. Because Kakashi did say that after that, he felt nothing. By taking revenge, it wouldn't make him feel any better.