As depicted in the visual, the sudden gun shots brought back memories of Vladeks childhood and how a dog had been shot down and killed in his neighborhood.
Thissymbolizes that even something like sound can trigger tragic memories from the past, which serves to foreshadow the current situation.
The dark and faceless picture of the dog rolling around in pain and dying and the a oaralell panel of a mouse dyig shows the difference of way is considered "human" and "animal". This classification is what defined the Nazi regime as well.
They were treated like "neighborhood dogs" and not people.
