Sergio Diaz was a feared, vicious and ruthless Mexican-American gangster, gang leader and mass murderer. He was one of the four main antagonists of the Season Four episode Lockdown.
He was the bloodthirsty leader and shot caller of the notorious Los Mags gang, considered one of the most dangerous Latino gangs in Los Angeles.
And he, his brother Adrian Diaz (Sergio's right-hand man and a high-ranking member of Los Mags) and two other members of the gang, Beni Rios and Nico Guzman, had been arrested a long time ago for executing nine people and because of that, it was expected they would get life without parole.
However, with nothing to lose, Sergio and the three members of Los Mags decide to escape from the court before the judge gives the sentence, which would end up leading to an unprecedented manhunt throughout Los Angeles, which only ended after an intense shootout between the four members of the Los Mags gang and the 20-Squad S.W.A.T. team, in which Sergio ends up being shot to death several times by Hondo and Deacon.