"Madero" was an alias assumed by Dr. Luis Nivara after he fled the United States and settled in San Macros, Mexico. In the States, he was indicted by a grand jury for insurance fraud because of testimony from Adrian Monk, although he and Monk never actually met each other due to it being a grand jury trial. Nivara lost his medical practice, and his wife committed suicide in humiliation. Nivara jumped bail and fled to Mexico, where he adopted his alias and became the coroner for the local police.
Nivara/Madero blamed Monk for "stealing" his life, and, because he was a wanted fugitive in the States, tried to lure Monk to Mexico instead. He killed a young man from a wealthy San Francisco family, faking evidence to make it look like he had been mauled by a wild lion - a seemingly "impossible" cause of death. This failed to lure Monk, so he tried again a year and a half later. This time, he killed Chip Rosati, another son of a rich, well-connected family, by creating another "impossible" death: a drowning in mid-air. This time, the mayor of San Francisco sent Monk to investigate. As Monk never actually saw him when he gave testimony against Nivara, he never recognized him when meeting him as Madero. He attempted to kill Monk three times, twice by running him over, and once by planting a bomb in Monk's hotel room. All three attempts were unsuccessful, and Monk eventually managed to solve the case and expose Madero. The Mexican authorities arrested Nivara. When Monk returned home, he received a letter from the police department in San Macros that said that they indicted Madero and there was a trial date.
It is unknown whether Madero/Nivara was convicted in Mexico, or extradited back to the States to stand trial there.