Board Thread:Legitimate Canon Questions/@comment-69.112.93.245-20140424235932/@comment-4091815-20140425013218

The simple answer is "because the writers of the show wanted him to die."

The only thing close to a canonical answer is that, as written, the sword was to puncture his heart and injure him in a way that even Werewolf healing couldn't fix.

This was imperfectly realized on screen so we're left with "just cause" as the answer to your question.