Problem: Reword: Dexter then realized that this was not true and that like him, Travis had a mental illness. To summarize, the Dark Passenger asks as a catalyst for Dexter's killing. Understanding the Dark Passenger allows us to learn more about why it is that Dexter kills. It is very important to understand a motive when these types of crimes are committed. As someone who had only seen the TV show, I thought Dexter was mentally ill, without a doubt. Though the novel presents a twist to the Dark Passenger and the concept of Dexter that is rather interesting to think about, I still believe that if Dexter was caught and interrogated, he would be diagnosed with a severe mental illness.