Board Thread:Show-Related Questions and Answers/@comment-27094327-20151019041819/@comment-27094327-20151020000224

Actually. it makes sense to me that he wants the dagger back. First, he wants to keep his perfect fake kingdom safe, as I think Eskaver said. What is the use of the Sands of the Avalon if the Dark One can just step in and take it all? Let alone if the Darkness can act of its own accord, as indeed seems likely, since Merlin contended with it in the distant past. Villains often have outsized ambitions, and one of Arthur's is to put his kingdom beyond any possibility of harm.

Second, he wants to kill Merlin, since he feels betrayed by him (and also because Merlin threatens his kingdom, as with the Darkness). He believes that Excalibur is powerful enough to do that, and that an ordinary weapon would fail. Putting aside the obvious flaws in attacking the most powerful sorcerer in existence with a weapon that he made and guided you to put back together, this is a fairly sensible villainous ambition.