Foreshadowing is an important literary device: Lennie kills mice (pets them too hard); Lennie kills his puppy (plays with it a bit too rough); Lennie kills Curley's wife (tries to make her stop screaming and accidentally breaks her neck). The mouse foreshadows the puppy, which foreshadows Curley's wife.
Again foreshadowing: George tells Lennie to "look around" when they are sitting at the river bed before they moved onto the ranch the next day. He told Lennie that if he got into trouble to come back to that very place. That "talk" George gave Lennie foreshadowed that Lennie would indeed get into trouble.
Once more, foreshadowing: Candy couldn't kill his own dog. Later on in the novel, Candy said that he should have killed his dog. This sad incident foreshadowed George making the decision to kill Lennie himself instead of let Curley do it.