It is a very good question. While I was reading this book an allusion to classical Greek tragedies came to my mind. A tragedy happens and the character involved takes to the open road. By the end of his journey, he undergoes a deep transformation and becomes a wise, kind and compassionate person. In this sort of transformation, i.e. metanoia, the person involved feels guilt and repentance that he overly admits and often refers to. I do not think 20th- 21st-century modern people are able to feel such profound guilt and repentance, however, some sort of guilt and repentance must be present in the process of maturing to become a better person.
Indeed, the novel does not show us whether McDowell feels pain and repentance or not. Probably there are certain events he regrets to have happened and probably he feels repentance, even guilt, over such events. Otherwise, he would have been an inhumane monster who is unable to change for the better. Fortunately, our McDowell has grown to a better man by the end of the story.