I think Robert felt responsible for Rita's death, in that he was the one who sent her to Paris with the art group. He ran away from his life because he could not stomach that he, Robert, killed her. Coleman's attitude toward the loss of life or harm of another was completely different. About to be cut off financially by his father, he practically celebrated the plane crash. So he certainly wasn't going to be bothered if, in fact, he did hit another person with his car. He didn't care about hurting or losing people he was related to, so a stranger was hardly worth his time. He made it clear that he was the only one who mattered.
I didn't understand Robert's "jealously of another man's bankroll" at this moment. To me, it is clear we have two different systems of justice; one for the wealthy and one for the poor. If Coleman had hit a person, he could have used his social standing or his money to absolve himself, but I think Coleman would have had the same reaction to the crash even if he was an average Joe. He simply was not a kind, thoughtful, moral person regardless of his bank account. I think Robert was tired of being around thoughtless, selfish people.
The most important part of this chapter for me was that in the face of Coleman's unwillingness to be responsible for his actions, Robert finally accepted that he WAS responsible for all of his that led him to this very moment. That is why he looked in the ditch. That is why he sat with the armadillo until it died. That is why he knew he needed to return to his wife.