There are many reasons the murderer might have dressed up the judge, and they are not explicitly stated. However, the reader can assume that the murderer dressed up Justice Wargrave so that his death reflected the pattern of the soldiers’ deaths in the nursery rhyme.

Recall the nursery rhyme about the ten little soldier boys. In the rhyme, each soldier dies in a different way. Up until this point in chapter 13, each person on the island has died in an eerily similar way and in the same order as the soldiers in the rhyme: for instance, Miss Brent died because of a prick in her neck with a bumblebee in the room. And to add to the mystery, every time a character dies, one of the ten figures of the soldiers mysteriously disappears. Now that there are five people remaining, it is natural that the next death will mimic the death of the fifth soldier. When there are five soldiers remaining, the rhyme states,

Five little soldier boys going in for law; One got into chancery and then there were Four.

To “get into chancery” means to dress up like a judge. Thus, in dressing up Justice Wargrave, the murderer is staying true to the rhyme. This odd parallel between the murders of the characters and the rhyme increases the tension and fear of those remaining on the island. It also makes the story as a whole more mysterious and suspenseful for the reader.