Why does Johnny react so violently to the Socs?

Johnny reacts so violently to the Socs because they are known for preying on Greasers. At the beginning of the book, we learn that Ponyboy was physically assaulted by members of the Socs, and Johnny himself has been a victim of the Socs' violent attacks. In chapter 2, Ponyboy tells the story of how some Socs members almost beat Johnny to death.
In Chapter 4, after Ponyboy's quarrel with Darry, Ponyboy goes to the park to cool off. He meets Johnny there, and before long, Randy and Bob (members of the Socs gang) corner the two. There are three other Socs with Randy and Bob. A Soc named David tries to drown Ponyboy in the fountain, and Ponyboy does his best to fight back.
However, Ponyboy is no match for David, and he soon suspects that he is going to die. As Ponyboy begins to lose consciousness, the pressure suddenly eases, and he finds himself on the ground beside the fountain. Ponyboy soon notices that Johnny seems dazed; the latter keeps telling Ponyboy that he's killed someone. 
It turns out that Johnny has killed Bob, one of the Socs. Johnny confesses that the Socs were going to drown Ponyboy and that he had to act before it was too late. So, Johnny reacts violently to the Socs because the gang regularly targets Greasers, especially if they are alone. He clearly remembers his attack at the hands of Socs months ago. Basically, Johnny's violent reaction stems from his fear that the Socs are aiming to kill him.