Paladin/Bards are allowed. Check out the Devoted Performer (I think that's the one?) feat in CAdv. In fact, with the Initiate of Milil, Divine Might, and Snowflake Wardance feats, it's a very playable combination.

1. Devils (and demons) want to - need to - corrupt mortals. The best way to corrupt someone is to offer them something they think they want, but with strings attached. What do you think a "deal with the devil" is all about, eh?

2. Who says fiends want to end the world? They just want to corrupt more and more mortals, and cause fear and pain and destruction in the Material Plane.

Fiends are creatures of philosophy, metaphysics, and belief. The more their ideals (pain, fear, violence, tyranny, chaos, terror, death) are present in the Material World, the more powerful they are (or feel).