Tony Soprano killed Christopher Moltisanti in The Sopranos because he believed it was necessary to protect others from the destructive influence of Christopher's drug addiction and reckless behavior. In the episode "Kennedy and Heidi," Tony and Christopher are involved in a car accident, during which Christopher admits that he is unable to pass a drug test. This revelation, along with Christopher's history of violence and instability, leads Tony to make the decision to kill him.

The show had dropped subtle hints about Christopher's fate leading up to his death. In the episode "Walk Like a Man," Tony expresses his frustrations as a parent and his concerns about passing on "rotten" genes to his son A.

J., who is struggling with depression and suicidal thoughts. Tony's interactions with his son and his therapist, Dr. Melfi, reveal his guilt and sense responsibility as a father. These themes foreshadow Tony's ultimate decision to eliminate Christopher, whom he sees as a threat to the lives of others.

Sopranos also uses symbolism and music to convey Tony's mindset leading to Christopher's death. The song "Comfortably Numb" Pink Floyd plays before the car accident, mirroring the emotional turmoil experienced by both Tony and Christopher. The lyrics "the dream is gone" underscore the central message of the scene, suggesting that Tony sees killing Christopher as a merciful act to spare him from a more tragic fate.

Furthermore, Christopher's role as a narrator in The Many Saints of Newark adds weight to his death. His narration provides context for the movie's title and highlights the impact his relationship with Tony on the DiMeo crime family. Christopher's supernatural presence in the prequel film suggests that he understands why Tony killed him and accepts his fate.

In retrospect, Christopher's death can be seen as justified due to his violent actions and destructive lifestyle. Throughout the series, he was responsible for the deaths of multiple individuals and showed little remorse. His drug addiction and the incident where he accidentally killed a dog while high on heroin further demonstrate his reckless behavior.

Michael Imperioli, the actor who portrayed Christopher, has discussed the emotional impact of the scene and its significance in the larger context the show. He noted that while filming the scene, it felt like "another day at work," but the true emotional impact hit him after the episode aired. Imperioli also highlighted the symbolic elements surrounding Christopher's death, such as his choice to wear a baseball cap in both his introductory and death scenes, as well as the presence of a crow, which was seen as a bad omen in a previous episode.

Overall, Tony Sano's decision to kill Christopher Misanti in The Sopranos was driven by a combination of factors, including his concerns as a parent, Christopher's destructive behavior, and the need protect others from the consequences of Christopher's actions.