So just what was Belial's plan there?

[Lissa]

All of this is bad writing, in part because it isn't simple. There is no clear arc, no obvious quest to keep us invested in the twists and turns. The micro-writing is fine - characters sound right, they make sense in immediate context - but the big picture isn't very strong.

-Jester

And this is why I've been saying for a long time that Blizzard needs to pull Metzen off doing any stories. He might have been alright (not good, just meh) back in the days of SC 1, WC 2/3, but he's gotten worse and worse. As I've noted in another post, Act 1 was alright (again, not good, just meh), but the overall story dies a horrible death as you go further and further into the game.