Whose death do you think was the most unfair?

It would have been, but not as interesting. In theory, Tieru's spirit could have appeared to Nicholas just as it did to Agrail, and then Nicholas, like Agrail-Railag, would have done everything to save the world because he loved Isabel just as much as Agrail. Then (if Agrail had been killed), the main heroes of the campaigns for Inferno and Dungeon would simply have been swapped. In my opinion, they would have been Biara and Tralsai.