"That was how the system worked. Knox's father purchased the boy's debt and when Knox broke the rules, the boy was punished." (pg58) Due to the class system Sydney Carton who is Knox's proxy must endure extreme punishments for his entire life. He happens to have a virus in his blood that could bring about the class systems demise. He has no desire to do this.
"Yes, Knox, and I hope this will teach you a lesson. Marie died."(pg56) Knox's reckless, selfish, and arrogant behavior due to the invincibility that the upper city class holds. Gets a young girl Marie Alvarez killed in a car accident after he breaks several laws. Of course this is the first time his actions will have a consequence on him.
"All will be forgiven. Even you. Even me."(pg 377)The class system is brought about by the virus in syd's and now Knox's blood. Knox sacrifices himself so that Sydney may live his life like he's always wanted to. He brings about the end of the system that once made him feel invincible. their is no longer an upper or lower class. Only Humanity.