Giovanni Brusca, who was convicted for the murder of Sicilian anti-mafia judge Giovanni Falcone, has been released after 25 years behind bars.
The news of his early release for good behaviour sparked a wave of emotion in Italy.
The newspaper La Republicca headlined "Brusca freed, the cruellest boss".
"In human terms, this news hurts me, but it is the law, a law that my brother wanted and that must be respected," judge Falcone's sister, Maria, told the daily.
Brusca, 64, was one of the closest collaborators of Toto Riina, the head of Cosa Nostra, the Sicilian mafia. He was arrested in 1996 and found guilty of activating the remote control on May 23, 1992, that detonated a bomb under Falcone's car near Palermo. The judge, his wife, and three of his police escorts were killed.
He was imprisoned in the Roman prison of Rebibbia and collaborated with the authorities, testifying in numerous trials. According to Italian media, he also admitted taking part in hundreds of murders.
Brusca was also behind the 1993 kidnapping of 12-year-old Giuseppe Di Matteo, whose father had turned against the mafia. He was kept in unspeakable conditions for two years and strangled. His body was then dissolved in acid. According to the police, this was "one of the most heinous crimes in the history of Cosa Nostra".
He will remain under probation for four years.
Tina Montinaro, whose policeman husband was killed in the 1992 attack, said she was "outraged" by Brusca's release.
"The state is against us, 29 years later we still don't know the truth and (...) the man who destroyed my family is free," she said.
His release was also criticised by many political leaders, both on the right and the left. The leader of the centre-left Democratic Party (PD) Enrico Letta called it "a punch in the stomach (that) leaves one speechless, wondering how it is possible".
"It is impossible to believe that a criminal like Brusca could deserve any favour. His release from prison is chilling," commented former European Parliament president Antonio Tajani, national coordinator of Silvio Berlusconi's right-wing party Forza Italia.
"A person who committed these acts, who dissolved a child in acid, who killed Falcone, is in my opinion, a wild beast and cannot get out of prison," Matteo Salvini, leader of the far-right League party, said.