No, I believe that Agrael killed Nicholas, after all. If it had been Biara, they would have definitely shown it to us. I wanted to say that Railag seems to have placed all the blame on Biara, even though she was completely innocent here (it's amazing: Biara killed so many: Beatrice, Tieru, Andrias, and Godric; and she was blamed for something she was actually not guilty of).
Added 7 minutes later
After all, Nicholas commanded the entire Necromancer army, which would have unconditionally followed him against the Overlord, which means he could have brought much more benefit than Faydaen, and Nicholas himself was stronger than Faydaen.