“He, I say—I cannot say, I. That child of Hell.”
In other parts of his same letter, Jekyll speaks of Hyde as a separate being (“that child of hell”), showing Jekyll’s shame over his evil actions. By speaking of Hyde in the third person (“He, I say—I cannot say, I”) he shows that he cannot bring himself to admit to the dreadful deeds he has done and seeks to distance himself from the immorality of his crimes.