By the time Euphemia is murdered, C.C. and Lelouch have made themselves very well known to Suzaku. After her death, Suzaku decides to retire; Euphie was the one light in his life, and with her snuffed out by the city’s pure evil, he can’t take it anymore. He also felt that retiring would be the best way to honor her even in death, but C.C. and Lelouch pay him a visit. They need him back, they decide, because with Zero gone, Schneizel el Britannia is allowed to do as he pleases with the city’s justice system. He’s been cleaning up the streets with Zero’s absence, all the while hiding the fact that he’s been collecting these criminals, from the petty crook to complete psychopaths, in order to create his own mercenary army. C.C. and Lelouch present to Suzaku the man responsible for Euphie’s death, primed for vengeance.
Suzaku killing him – the first and last man he ever killed – marks his transition to a much darker version of Zero.