Hedwig's killer was a Death Eater that fought in the Battle of the Seven Potters. This Death Eater shot out a Killing Curse that killed Harry Potter's pet owl, Hedwig. It is likely the curse that killed Hedwig was intended for Rubeus Hagrid. He most likely took part in the Battle of Hogwarts. It is also likely that, after Lord Voldemort's defeat in 1998, this Death Eater was either killed or imprisoned.