In their first appearance, the Cousins plan on assassinating renowned meth cook Walter White - also known as Heisenberg - as retribution for betraying Tuco and causing his death.
Why did the twins go after Hank? Because they knew that Hank was the one who killed their cousin Tuco Salamanca. Although Walter was involved in his death, but Gus manipulated them which led them trying to kill Hank.
As members of the Salamanca family, the two cousins are grandsons of Abuelita and the nephews of Hector Salamanca. Marco and Leonel also have three cousins who are involved in criminal activities: Tuco, Lalo, and Joaquin.
They learned of the death of their cousin, Tuco Salamanca, so they targeted Heisenberg without knowing his identity. After crossing into the States, the bothers paid a visit to their uncle, Hector Salamanca, who used a Ouija board to unveil Walter White as the man they were looking for.
Seeing Walt break in through the crawl space, Mike heads back to his car. However, Mike notices another car pulling up; Marco and Leonel step out and, with a large mirror-polished silver axe in hand, enter Walt's house.
Gus Fring tipped Hank off. Even though he did send the brothers to attack Hank in the first place, he warned Hank so that he would be aware of what's going on and proceed to kill them, thus wrapping up a loose end.
Off camera, Walt crushes up some of the poison from the Lily of the Valley plant and injects it into a juice box. Also off camera, Walt delivers the juice box to Brock's school.
Bryan Cranston was not the first choice for the role of Walter White. In fact, casting him took some convincing by the show's creator and writer. At the show's inception, the role of Walter White was initially offered to actors Matthew Broderick and John Cusack.
Lalo Salamanca is one of the cartel's most feared members, known for his charm, strength, and business savvy. He is the nephew of Don Hector Salamanca and the cousin of Tuco Salamanca, who are prominent characters in both Better Call Saul and its father series Breaking Bad.
Simple, complicated, it doesn't matter. Steps never change, and I know every step.
Lalo is one of many nephews of Hector Salamanca, an enforcer of Don Eladio Vuente's drug cartel.
A member of the Salamanca family, Tuco is the grandson of Abuelita and the nephew of Hector. Tuco also has four cousins who are involved in criminal activities: Lalo, Marco, Leonel, and Joaquin. He is the first Salamanca family member to run their drug operation, and was succeeded by his uncle Hector and cousin Lalo.
It was probably to show humility to their god. In a later episode, one of the cousins had his legs amputated and while being treated at the hospital, he dismounted from his bed and crawled toward Mr White in a feeble attempt to kill him.
Gus (or one of his men) called Hank in order to give him a chance to defend himself. This was Gus' way of almost sticking it to the cartel without being too obvious. If Gus ordered a hit out on the brothers himself, it's likely the cartel would know and declare war on Gus and his operation.
Hector Salamanca
Having served as one of Don Eladio's most feared men for decades, Hector has been as close to the top as it gets.
Hector Salamanca, also known as Don Hector, was the elderly don of the Salamanca drug organization and an associate of cartel boss Don Eladio Vuente. He was the son of Abuelita, the uncle of twins Marco and Leonel Salamanca, Tuco Salamanca, and Lalo Salamanca, and the grandfather of Joaquin Salamanca.
Walt, the trained scientist, calls himself “Heisenberg” after the Heisenberg Uncertainly Principle by the German physicist Werner Heisenberg, who posited that the location and momentum of a nuclear particle cannot be known at the same time.
Throughout the series, Marie is almost always shown wearing the color purple, which creator Vince Gilligan explained is symbolic of her being misled; for example, Walt and Skyler were deluding Marie about who was behind the drug business.
As Saul goes to grab his gun, Jesse reaches it first and points it at the bloodied lawyer. He wants him to admit that he stole the ricin cigarette to help Walt. Saul pleads with Jesse.
1. Brock obviously recognized and was scared of him when he went to Andrea's house in the last season, but based on his actual reaction to Walt, I don't think Brock knew for certain that Walt had been the one who poisoned him. Otherwise he would have reacted much more violently to Walt's presence in his home.
Long story short, Walt killed Lydia to protect Skyler and the kids. She was presumably the only Madrigal employee still working on the meth operation.