Giordano Bruno

Giordano Bruno, born Filippo Bruno, was an Italian mathematician, philosopher, and astronomer. Oh, and also a Dominican friar. He was burned at the stake in 1600 by the Inquisition for being a pantheist, and partly due to his expansion of Copernican theory in which the sun was merely a star and that there were an infinite amount of worlds inhabited by intelligent beings, despite that many of those same cosmological beliefs had been held by Cardinal Nicholas of Cusa. After his death, he gained considerable fame, being particularly celebrated by 19th- and early 20th-century commentators who regarded him as a martyr for science, although most historians agree that his heresy trial was not a response to his cosmological views but rather a response to his religious and afterlife views

Religious views
Bruno believed that each race had its own Garden of Eden, be it those on other worlds and those on Earth. He also believed that God was not divine, but simply a very talented magician, and that Satan will eventually be saved. Needless to say, these views weren't exactly mainstream.

Execution
On the way to the stake: As the parade moved on, Bruno became animated and excited. He reacted to the mocking crowds, responding to their yells with quotes from his books and the sayings of the ancients. His comforters, the Brotherhood of St. John, tried to quiet the exchange, to protect Bruno from yet further pain and indignity, but he ignored them. And so after a few minutes the procession was halted by the Servants of Justice. A jailer was brought forward and another two held Bruno’s head rigid. A long metal spike was thrust through Bruno’s left cheek, pinning his tongue and emerging through the right cheek. Then another spike was rammed vertically through his lips. Together, the spikes formed a cross. Great sprays of blood erupted onto his gown and splashed the faces of the brotherhood close by. Bruno spoke no more. … as the fire began to grip, the Brothers of Pity of St. John the Beheaded tried one last time to save the man’s soul. Risking the flames, one of them leaned into the fire with a crucifix, but Bruno merely turned his head away. Seconds later, the fire caught his robe and seared his body, and above the hissing and crackling of the flames could be heard the man’s muffled agony.

Vatican position
In 1942, the cardinal who rediscovered the trial documents said the church was perfectly justified in executing him. In 2000 another cardinal said the event was "sad" but still supported the executioners.