RTF is a global modification that replaces all towns in the game.

First, why is it "lawful"? Second, a rain of arrows + minion control + berserk – isn't that a bit too overpowered?

Because a light-dark human looks more realistic than just a light-cute faction, which would be better suited for someone else, someone more fantastical and less realistic.
The human's right to darkness is a right to color, atmosphere, and depth, so to speak. It's a right to be flawed, rather than just fanatical saints. A right to think and choose, rather than follow a single, correct path.