The author creates suspense in many ways in this story. The first way is through the overall situation: the man who killed Frank is out on bail. What's worse, the parents keep running in to him. That's suspense in itself: what will happen? Then more factors are added. Matt buys a gun, making it more likely that things will happen.
When Matt can't take it anymore, he goes to where Richard (the man who killed Frank) works—but instead of just killing him, he orders him into a car at gun point. This creates an unstable situation, and considerable tension. The picture of Mary Ann at Frank's place raises the possibility that another person might arrive, introducing another factor for tension.