My thoughts are this.
Imperius becomes fed up with the Nephalem and their sudden rise to power. He decides to unite all the angels that share his fear/hatred for the Nephalem. He decides to unbanish Lilith hoping she will join him in his fight because it has been so long.
Lilith initially tells Imperius she will help and then betrays him becoming the "Tyrael" (non human that assists us in our quest) in the expansion. Assisting and guiding the players to defeat Imperius because her love for her "children" is more than her love for Imperius.
Could even have a fight early on between your character and Imperius/Lilith in which she betrays him and sides with us but can't kill him. He escapes and she decides to help us to kill him because she knows she can't do it herself.