Board Thread:Legitimate Canon Questions/@comment-4091815-20131214163949/@comment-4091815-20131225155521

There is actually a question in the canon about who did the sliceing. We know Peter killed her. That's how he became the alpha.

As to who sliced her in twain. There is some suggestion that the Argents did that but the prevailing theory is that Peter did it, after gaining his alpha status, to make it seem her death was hunter related.

Either way, her death and the condition of the body drew Derek back to town and caused him to blame the Argents.