I ought to of shot that dog myself, George. I shouldn't ought to of let no stranger shoot my dog.
George and Lennie are discussing their dreams of living on their own with cattle and rabbits. They both have this same dream in mind. Their ideas are expressed with imagery, this is useful because it gives the reader a clear image of their goal. The theme of this scene is to not be too quick to believe in a dream.
I done a real bad thing
Here Carlson killed Candy's dog because the dog was old and in a lot of pain. Candy was mourning his dog in his bed. This is foreshadowing future events because later in the story Lennie is compared to the dog as old and useless and consequently gets the same fate. The theme of this scene is that there is no room for burdens.
No, Lennie i ain't mad.
Lennie broke Curley's wife's neck and killed her, he also had killed a dog before and tried to hide it. Conflict is shown is this scene, Lennie has killed two living things and now really bad consequences will a rise. The theme is to not ignore the inevitable.
I thought you was mad at me, George.
George shoots a bullet in Lennie's head and kills him. Before his death they are talking for a bit about what Lennie had done to Curley's wife. In the background there is a heron eating a snake representing George and Lennie. The Heron killing the snake symbolizes good killing evil, similar to George killing Lennie. The theme is destroy evil before it becomes too powerful.