Lennie is hiding a dead mouse that he had killed from George. The author is foreshadowing Lennie killing Curley's wife during this scene. This scene reveals the theme, plans never go to according to the way you made them.
Sure right now, I gotta, we gotta.
Carlson is taking Candy's old useless dog to put it down. It's foreshadowing Lennie getting killed because he is also useless. The theme shown at this moment is usefulness is necessary to be respected.
Lennie is sitting in front of the young dog that he killed, this foreshadows Lennie killing Curely's wife in the same chapter. The theme in this chapter is you can never be to careful.
George is killing Lennie because of what he did to Curely's wife. This scene is an allusion to the poem "To a Mouse". The theme in this scene is the same as the first, plans never go according to how you had imagined they would.