In Captain America: Civil War (2016), Black Panther sided with Iron Man to exact revenge on the Winter Soldier, but there was a bigger reason.
When the Marvel Cinematic Universe decided to adapt the Civil War story from the comics, they presented a difficult situation for movie fans, as they were asked to choose sides between Iron Man and Captain America. However, to make everything seem more spectacular they presented new heroes such as Spider-man or Black panther.
Some of the reasons the characters sided with Iron Man or Captain America made sense. Spider-man I'd do anything for Tony Stark Ant-Man he had complete respect for Captain America, just like Falcon. However, Black Panther's reasons for joining Iron Man's side seemed personal: he believed that the Winter Soldier, who was fighting Captain America, was responsible for the terrorist attack that killed his father, the King T’Chaka. However, there may be more than just revenge.
The Civil War between the Avengers wasn't about the Winter Soldier, it was about the Sokovia Accords.
These were implemented after the devastation heroes wreaked on the fictional nation of Sokovia during the events of Avengers: Age of Ultron. The Sokovia Accords were meant to hold anyone with superpowers accountable, including the Avengers. Steve Rogers' refusal to sign the deal is what ultimately sent him underground, where he created the MCU version of the Secret Avengers. However, these Agreements also established a reason for Black Panther to side with Iron Man in Civil War, even if there had been no reason to exact revenge on the Winter Soldier.
The Accords say: "Any enhanced individual who signs is prohibited from taking action in any country other than his own, unless he first receives approval from the government of that country or a subcommittee of the United Nations."
This is important to note because both T’Challa and his father T’Chaka before him led the African nation of Wakanda. Black Panther established how important it was to keep the technological advancements of the Kingdom of Wakanda secret. The Accords ensured that no hero from outside of Wakanda could act within the country without approval, protecting the nation from a superhero who steps in to try to help, learn the truth, and expose Wakanda to the world.
Then he changed his mind.
At the end of the Black Panther movie, T’Challa realized that he needed to open Wakanda to the world to help other countries move forward. However, during Captain America: Civil War, T’Challa was a new king and, to continue his father's legacy, he would feel compelled to continue to keep Wakanda hidden. After all, T’Chaka came to Lagos to defend the Sokovia Accords, and T’Challa would want to honor his father's wishes after his unexpected death. Consequently, Black Panther joined Iron Man's side in the Avengers battle to protect his nation first and seek revenge against the Winter Soldier second. This would explain why he didn't kill Zemo when he got the chance. Since he knew that revenge would destroy him inside.
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