LÃ©on: The Professional

LÃ©on Montana (Jean Reno) is an Italian hitman (or "cleaner", as he refers to himself) living a solitary life in New York City's Little Italy. His work comes from a mafioso named Tony (Danny Aiello). LÃ©on spends his idle time engaging in calisthenics, nurturing a houseplant, and watching old films.
One day, LÃ©on sees Mathilda Lando (Natalie Portman), a twelve-year-old girl who is smoking a cigarette and sporting a black eye. Mathilda lives with her dysfunctional family in an apartment down the hall. Her abusive father and self-absorbed stepmother have not noticed that Mathilda stopped attending class at her school for troubled girls. Mathilda's father (Michael Badalucco) attracts the ire of corrupt DEA agents, who have been paying him to stash cocaine in his apartment. After they discover he has been cutting the cocaine to keep some for himself, DEAÂ agents storm the building, led by sharply dressed drug addict Norman Stansfield (Gary Oldman). During the raid, Stansfield quickly becomes unhinged and murders Mathilda's entire family one by one while Mathilda is out shopping. When Mathilda returns, she realizes what has happened just in time to continue down the hall, where she desperately knocks on her neighbour's door. A hesitant LÃ©on gives her shelter.
Mathilda quickly discovers that LÃ©on is a hitman. She begs him to take care of her and to teach her his skills, as she wants to avenge the murder of her four-year-old brother. At first, LÃ©on is unsettled by her presence. He even almost kills her in her sleep, possibly out of mercy, but can't bring himself to do it. The next morning, LÃ©on tries to throw Mathilda out, but she claims that, because he saved her life the previous day, he is now responsible for her, stating that she would be dead by the end of the day if he threw her out and it would be like he had never opened the door for her. She begs him to train her as a "cleaner," demonstrating her willingness to learn by taking LÃ©on's gun and firing randomly onto the street outside his open window. Reluctantly, LÃ©on trains Mathilda and shows her how to use various weapons. In return, she runs his errands, cleans his apartment, and teaches him how to read. Mathilda tells LÃ©on she loves him several times, but he offers no response.
One day after Mathilda has learned how to shoot, she fills a bag with guns from LÃ©on's collection and sets out to kill Stansfield. She bluffs her way into the DEA office by posing as a delivery girl, only to be ambushed by Stansfield in a bathroom. Mathilda learns from Stansfield that LÃ©on killed one of the corrupt DEA agents in Chinatown that morning. LÃ©on, after discovering her plan in a note left for him, rescues Mathilda, shooting two more of Stansfield's men in the process. A now enraged Stansfield goes to find Tony, and assaults him in order to find out where LÃ©on is.
When Mathilda returns home from grocery shopping, an NYPD ESU team sent by Stansfield captures her and attempts to infiltrate LÃ©on's apartment. LÃ©on ambushes the ESU team and grabs Mathilda. Back in his apartment, LÃ©on creates a quick escape for Mathilda by smashing a hole in an air shaft. He reassures her and tells her that he loves her and that she has given him "a taste for life", moments before the police come for him. In the chaos that follows, LÃ©on sneaks out of the building disguised as a wounded ESU officer. He goes unnoticed save for Stansfield, who recognizes him, follows him downstairs and shoots him in the back. As he is dying, LÃ©on places an object in Stansfield's hands that he says is "from Mathilda". Opening his hands, Stansfield discovers that it is the pin from a grenade. He then opens LÃ©on's vest to find a cluster of active grenades, which detonate moments later, killing them both.
Mathilda heads to see Tony, as LÃ©on had instructed her to do before he died. Tony reveals to Mathilda that LÃ©on instructed him to give his money to her if anything happened to him. He offers to hold the money and provide it to her on an allowance basis, on account of her youth. Mathilda asks Tony to give her a job as a hitman; Tony adamantly refuses to hire a twelve-year-old girl, telling Mathilda to forget about her ordeal and return to school. Mathilda meets with the school headmistress, who readmits her after Mathilda tells her what had happened in the past several days and if the headmistress doesn't help her now, Mathilda would be dead by the end of the day. She then walks into a field near the school to plant LÃ©on's houseplant, as she had told LÃ©on he should, to give it roots.

Question: Whose death does Mathilda want to avenge?
Answer: her brother's