It was what Rowland calls Bruno's "combative personality" that finally did him in. The Roman Inquisition, in an especially insecure and punitive mood on account of widespread Protestant agitation against the church, had only the Venetian nobleman's testimony against the philosopher. Then one of Bruno's former cellmates, a man he'd slapped during a dispute and who feared that Bruno had informed on him as well, stepped forward to relate the various blasphemies and heretical convictions Bruno had spouted during their time together behind bars.
Their fellow prisoners confirmed that Bruno had cursed God, Christ and the church. Of course, many Italians (then and now) have been known to do this in moments of pique, but the Inquisition also had ample evidence of the philosopher's contempt for friars, Jesuits, scholastics and other church figures (not to mention his very real objections to key Christian doctrines) in his printed works. He had vented as much bile as the most virulent Internet troll, but he was much more eloquent and far from anonymous. Eventually, he ran out of friends and second chances.
The last straw was Bruno's refusal to accept the authority of the Inquisition itself. Even so, his rebellion was peculiarly Catholic: He kept insisting he'd recant if the pope personally confirmed to him that his beliefs were heresy. This infuriated Cardinal Bellarmine, known for his conviction that harsh punishments make good teachers. Sixteen years later, Galileo managed to elude the more extreme penalties meted out by Bellarmine and company with a public (and essentially politic) repudiation of his heliocentric views; he lived to fight another day under a relatively comfortable house arrest. Bruno was characteristically less prudent, and died naked and gagged (by some accounts with an iron spike through his tongue), in flames.
As Rowland points out, Bruno, irascible as he was, had committed no crime, not even the disruption of mass, a common practice by militant Protestants of the day (and also punishable by death). He "had done nothing in his life except talk, write and argue." When his fate was pronounced, he told his condemners, "You may be more afraid to bring that sentence against me than I am to accept it."