The justices ruled 4-1 today that 42-year-old Donney Council's lawyer should have done more than simply have Mr. Council's mother testify during the sentencing phase of his trial.
Mr. Council's current attorneys say that his father was a violent alcoholic, that he suffered two head injuries as a child and that there was extensive mental illness in his family.
Authorities say Mr. Council raped Elizabeth Gatti in her home, then forced her to ingest cleaning fluids before killing her in October 1992. Mr. Council's conviction still stands.
The state's attorney general's office says it will ask the state Supreme Court to reconsider its ruling.