Who does David think committed murder and how does David feel about it?

In Chapter 18, Alan woke first and spoke to David. David however refused to lift his head and told him that they must part company. Alan demanded to know why and David explained that he did not wish to be near a man who either participated in or committed a murder. By describing how foolish it would be for Alan to take part in a murder for his safety and his clan's, Alan was able to convince David that he had no part in the crime. He was less successful in answering if he knew the identity of the murderer but David appreciated his skillful avoidance of the question and his loyalty to his own code of morals. David thought Alan immoral when he threw the soldiers off the murderer's track but was nevertheless quieted.