Instead of these gory tales, the courtroom audience had expected a prosecution witness, Jesus Zambada García, to confess to a bombshell act of corruption. Last week, in a private sidebar conversation, Mr. Guzmán’s lawyers said that Mr. Zambada would testify to having paid at least $6 million to the “incumbent president of Mexico.” That testimony never came on Monday, but it may on Tuesday when Mr. Zambada returns to the stand.

Image The prosecution’s star witness in the current trial of El Chapo, Jesus Zambada García, was first presented to the Mexican press in 2008 shortly after his arrest. Credit... Alexandre Meneghini/Associated Press

But no less than a half-dozen times on Monday, Mr. Zambada, said that Mr. Guzmán, his former boss in the Sinaloa drug cartel, had arranged for people to be killed for seemingly minor reasons. In what was Mr. Zambada’s third day as a witness, Mr. Guzmán emerged as a kind of gun-loving hothead who owned a diamond-encrusted pistol with his initials on the handle and who once relaxed by taking target practice with a bazooka.

From the start of the trial, which is being held in Federal District Court in Brooklyn, prosecutors have accused Mr. Guzmán not only of earning $14 billion by routinely shipping ton-size batches of drugs into the United States, but also of taking part in more than 30 murders. Mr. Zambada began on Monday to describe some of those killings, starting with the slaying of Rodolfo Carrillo Fuentes, a former member of the Sinaloa cartel.

In 2004, he said, Mr. Guzmán and Mr. Carrillo Fuentes were rivals as part of a larger war between the Sinaloa traffickers and a vicious gang, the Zetas, with whom Mr. Carrillo Fuentes had allied himself. At a meeting to restore the peace, Mr. Guzmán’s longtime partner, Ismael Zambada García, tried to broker a truce. But when Mr. Guzmán put his hand out Mr. Carrillo Fuentes did not take it.

Not long after, Mr. Zambada said, armed assassins lay in wait for Mr. Carrillo Fuentes and his unsuspecting wife, gunning both down as they exited a movie theater in Culiacán. “Chapo said he was going to kill him,” Mr. Zambada told the jury.