What does Cust share with Poirot during their meeting in Chapter 33? The A.B.C. Murders

Cust tells Poirot about his history, about how his mom wanted him to be someone famous, but he was bad at everything in school. He went to the war and enjoyed being in it, but he got a wound in the head and had to return and become a salesman. He believes that he committed the murders, but he does not have an explanation for why.