Since your character falls under a lot of the hallmarks of "anti-hero", complicate the relationship between their brother, their friend, and their sense of revenge. If their brother and they were both betrayed by the system, but react differently, resulting in the death of the Main Character's Friend, who was more in alliance with the Main Character, then the character has a choice: Abandon or shift their revenge plot against their brother, or try to take a more forgiving stance. This will force your character to grow in an atypical way, since Heroes tend to be a bit one-note, predictable, and a tad boring. I've not read a lot of books that lean in this direction, and your concept sounds very interesting!
You could also make the brother's decision to kill the friend affect/effect (both words apply) how Main Character's plan to Change the System. In Black Panther, Killmonger's plans and ideology--though fundamentally flawed--do effect change through T'Challa, who shifts his worldview to include Killmonger's intentions. This is the first time in the Marvel Universe a villain has effected positive change AND changed the mind of a hero. Your book could take a similar approach.