In Chapter 2 of S.E. Hinton's novel The Outsiders, why was Two-Bit's trick on Johnny especially cruel?

Two-Bit Mathews is one of the Greasers in S.E. Hinton's novel The Outsiders. The Greasers are the boys and young adults from the poor section of town who are engaged in a perpetual state of war with the Socs, the boys from the upper end of the socioeconomic spectrum. That war, as all wars do, has taken its toll on its participants, some more than others. One of the casualties is Johnny, one of...

Two-Bit Mathews is one of the Greasers in S.E. Hinton's novel The Outsiders. The Greasers are the boys and young adults from the poor section of town who are engaged in a perpetual state of war with the Socs, the boys from the upper end of the socioeconomic spectrum. That war, as all wars do, has taken its toll on its participants, some more than others. One of the casualties is Johnny, one of the novel's main characters, a Greaser, and the closest friend of the story's narrator, Ponyboy Curtis. Two-Bit is, in Ponyboy's description, "the oldest of the gang and the wisecracker of the bunch." It is not surprising, then, that it would be Two-Bit who sneaks up on the two younger boys, Johnny and Ponyboy, who are engaged in conversation with two Soc girls, and frightens them into thinking that they are about to be attacked by the Socs:

"We were all four sitting there in silence when suddenly a strong hand came down on Johnny's shoulder and another on mine and a deep voice said, 'Okay, greasers, you've had it.'" 

Two-Bit's joke is particularly frightening to Johnny because the latter has recently been on the receiving end of a brutal beating at the hands of the Socs, and he has been skittish ever since. Johnny, in stark contrast to the other Greasers, is not particularly tough. In fact, he is a shy, frightened child who, in Ponyboy's words, "couldn't say 'Boo' to a goose." As a result of being beaten by the Socs, Johnny carries a knife that will be instrumental in one of the novel's more consequential scenes, when he kills a Soc who is drowning Ponyboy--a development that triggers the next sequence of tragic events while illuminating the courage this young boy still has deep inside himself. The cruelty of Two-Bit's attempt at humor, however, lies in the emotional scars Johnny continues to carry long after the physical scars have healed.