User blog comment:Zelron/Ageless bloodlust (How long until you're hungry?)/@comment-4057437-20121021075337/@comment-3492791-20121021080156

You need to look a little beyond face, mate.

Durnehviir is bound to guard the Soul Cairn until Valerica dies. Since she is undying, he can't wait her out. Thus he would jump at the chance to kill her the second he could, as it would fulfill his oath. Which in fact he did try to do.