I guess the issue that I have with that is that Tyrael is trying to convince Leah that her uncle's stories were more than just stories. She's ready to immolate Cain and the last collection of his work, essentially ending the Horadric Mage line forever.
From that revelation, she picks up the book and carries on with Cain's studies, trying to decipher the Black Soulstone and the associated end of the world lore.
From a literary sense, the bow is drawn, the arrow is nocked and Leah is pointed in the right direction. Well done. Then we hit this snag of Adria, then our heroine becomes Diablo...?
- So what was the point in her becoming the last of the Horadrim, if they just kill her off later?
- What was the point of having her change her mind at all if Tyrael already knew everything we needed to know?
- Does she ever discover anything of any importance that's not told to us by Adria or Tyrael first?
But I ramble... As for good one-liners, I like the one where you destroy the last siege engine. Azmodan, "You're still surrounded by the armies of Hell, blah, blah, blah..." You, "I've thrown your armies from the walls, driven them from the fortress, killed your lieutentant, destroyed your siege engines... and now I come for you!"