White Dragon Scenario Is Odd (Spoilers)

So to really start the main quest you have to storm the castle and kill everyone--without a super solid reason to? Like, you know a lot of people have been hung, but you have no idea if those people were murderers, thieves, or if Lord Bierwood is just silencing political dissidents. From what you find in the castle he's obviously evil but--how does the party know? Felt weird.