Okay so spoilers for the choices for the new Divine in Dragon Age: Inquisition.

There is a way to prevent the whole “rivers of blood in the Grand Cathedral” part that can happen with one of the options. And it’s not an obvious option.

To avoid the “rivers of blood” if the game gives you (or you choose) Leliana for Divine, you have to “un-harden” her. First, don’t let her kill the traitor in Haven. Then make sure that she doesn’t kill the person you run into on her personal quest. There’s a walkthrough video on that here.