Events leading to the death of Curley's wife

• We know that Lennie likes to pet soft things, and often kills them in the process (the mouse in his pocket, his pup). He doesn't know his own strength. 
  
• When Curley's wife offers to let him pet her soft hair, it is clear that he will end up killing her, based off of past events.

Events leading to the death of Lennie

• Candy allows Carlson to shoot his dog. He had raised this dog since it was a pup and simply let another man deal with it when it got too old for its own good. Candy even states later on that he shouldn't have let   someone else shoot his dog. 
• This foreshadows that George will end up shooting Lennie. He is too good of a friend towards Lennie to let anybody else do the job.