Angel, the son of Julia's supposedly blind neighbor, is behind all the deaths.
He killed the real Ivan.
He blinds his victims, and then, only kills them if they reject him. He assisted in the suicide of Sara and arranged the deaths of Issac and Créspulo. He also blinds his mother when he learns she can see again.
Angel hates to be ignored and finds that only blind people notice him and he can only now function if they rely on him for aid.
He subsequently slits his own throat rather then be arrested.
Issac donated his corneas to Julia to restore her sight, but the operation was a failure and she will still soon lose her sight.
Thanks N.D. Woods!