She slang her pups last night. Nine of 'em. I drowned four of them right off. She couldn't feed that many.
He won't even feel it.
Oh, Jesus Christ!
Slim was explaining to Carlson that his dog had 9 pups, but he had to kill 4 of them to keep the other 5 pups and the mother alive. This shows that Slim is willing to sacrifice something to get something better out of it. This also proves that sometimes sacrifices have to be made in order to get more out of a situation.
I see it George!
Carlson is taking Candy's old dog outside so he can shoot him because Candy couldn't get the courage to shoot his dog himself. However, killing something that is suffering ultimately ends it's suffering.
Curley's wife had her neck snapped by Lennie, and George and Candy found out. Now, everyone is going to be hunting down Lennie to make him suffer. However, we have to take responsibility for our mistakes, and Lennie didn't do that.
George finds Lennie, and kills him near the river where they started in the beginning of the book. George killed his only good friend, but he had to do it to prevent him from being tortured by the others. This shows that sometimes you need to sacrifice something to prevent it from suffering more.