Why do you think the death of Lennie's puppy is described after its death and not as it happened ?

There is a lot of foreshadowing in this story. This is one of those times where the death of the puppy foreshadows the death of Curley's wife. The death of the puppy does not come as unexpected. Lennie has inadvertently killed what he covets as soft and cuddly. Curley's wife is the next obvious victim. More than this, Steinbeck portrays death as release from the unfair reality that many of his characters live in.