His most famous speech comes in Chapter 28:
Whenever they's a fight so hungry people can eat, I'll be there. Whenever they's a cop beatin' up a guy, I'll be there . . . . I'll be in the way guys yell when they're mad an'--I'll be in the way kids laugh when they're hungry an' they know supper's ready. An' when our folks eat the stuff they raise an' live in the houses they build--why, I'll be there.
Tom has just left his family in order to protect them from harboring a vigilante. Instead of running from the law, he is going to fight against it. Tom is going rogue, becoming a ghost, a vigilante for social justice. He vows to be a kind of sacrificial lamb for the common man and working class heroes. Not exactly a Christ-figure (because Jesus was against violence), but certainly a martyr for the migrant worker. Tom is Steinbeck's tireless hero who sacrifices family, identity, and his American Dream to protect theirs.