Former judge Alberto Guerra Bastida leaves federal court in New York, on Oct. 22, 2013

Salvatore “Sammy the Bull” Gravano, a murderous underboss in the Gambino crime family, capped a colorful career by turning state’s evidence and helping to bring down John Gotti, aka “the Dapper Don.” Sometimes you need a bad guy to go after a bad guy.

That’s Chevron (CVX)’s approach in going after Steven Donziger, a New York plaintiffs’ lawyer who won a record-setting $19 billion pollution verdict against the oil company in Ecuador. Chevron claims Donziger engineered an international conspiracy to obtain the 2011 judgment, one that included blackmailing and bribing Ecuadorian judges; he denies those accusations. In the high point so far of an unusual trial that has already lasted two weeks, Donziger’s lawyers had a chance on Friday to cross-examine Chevron’s main witness, Alberto Guerra, a former Ecuadorian judge who has testified that he took bribes to tilt the proceedings in a rain forest courtroom against the company. Guerra is Chevron’s Sammy the Bull. The open question is whether the company can use him to bring down Donziger, who would be the Dapper Don in this scenario. (Yes, yes—no one has been accused of literally whacking anyone. Let’s not get too literal-minded.)

Chevron’s suit in federal court in New York seeks a judicial order barring Donziger or his Amazonian clients from ever profiting from the Ecuadorian judgment. Chevron has invoked the civil provisions of a 1970 anti-mob law, the Racketeer Influenced and Corrupt Organizations Act, or RICO, which also authorizes the federal judge in New York to force Donziger to pay tens of millions of dollars in Chevron legal fees if the company prevails.

Guerra, meanwhile, has admitted that he viewed his career on the Ecuadorian bench in the 1990s and 2000s as an opportunity to harvest illegal payments to fix cases. “While I was a judge, the payment of bribes to judges in exchange for a desired result was commonplace,” he said in a sworn written declaration accompanying his live testimony. “And as a judge, I occasionally accepted bribes from litigants in exchange for issuing favorable rulings.” He also admitted that as a practicing lawyer, he paid bribes to help his clients.

In Donziger’s suit in Ecuador against Chevron, Guerra said he played a more nuanced, though equally dishonest, role: that of ghostwriting rulings issued by another judge that favored Donziger’s clients. Guerra testified that he did this in exchange for envelopes of cash handed over by one of Donziger’s Ecuadorian lawyer colleagues. Guerra also claims that he negotiated a deal under which Donziger’s team agreed to pay $500,000 for the opportunity to draft the 2011 judgment. Donziger has acknowledged meeting privately with Guerra but denies all the former judge’s other allegations.

Under sharp and at times disdainful questioning by Zoe Littlepage, one of Donziger’s attorneys, Guerra admitted in federal court on Friday that he has told slightly different versions of his story at different times. When first discussing his cooperation with Chevron in 2012, Guerra told company representatives that Donziger’s team promised to pay him $300,000 to help steer the case in Ecuador in their favor. “It was an exaggeration on my part to secure a better position for myself,” Guerra said on the witness stand, via a translator. “It was not true.”

If Littlepage and her client thought they were going to rattle Guerra, however, they were incorrect. Jut-jawed and defiant, the disgraced former judge matter-of-factly described his misdeeds as nothing more or less than the way legal business gets done in Ecuador. He waved off Littlepage’s skepticism and dismissed his past inconsistencies as unimportant details.

Guerra confirmed that Chevron has paid him tens of thousands of dollars in cash—nominally as compensation for turning over his computer, cell phone, and other material—and has resettled his extended family in the U.S. Chevron has committed to supporting Guerra for two years, paying for his car, home, legal representation, and even health insurance.

The ultimate arbiter of whether Guerra is now telling the truth is Lewis Kaplan, the U.S. district judge hearing Chevron v. Donziger without a jury. In a series of pretrial rulings, Kaplan has indicated that he takes the evidence against Donziger seriously. Chevron has presented bank records, diary entries, and other material it says corroborate Guerra’s account. The company has also unearthed suspicious passages in the 2011 judgment that repeat, word for word, parts of plaintiffs’ memos that were never filed with the court in Ecuador.

Chris Gowen, a spokesman for Donziger, argued that Chevron’s reliance on Guerra ought to raise questions about whether anything the company says deserves to be taken seriously. “He’s the worst, least believable witness who has ever testified in federal court,” Gowen said outside the courtroom. Hmm. Worse than Sammy the Bull?

The trial is scheduled to resume on Thursday and continue through much of November.