Musar sur Les Nombres 22:34

חטאתי כי לא ידעתי . At first glance it seems strange that if Bileam was not aware of the angel's presence, (which had impeded the she-ass's progress), how he could have sinned. Man is responsible for ignorance of certain laws however, if they were the ones he should have known. Otherwise there would be no point in G–d equipping us with a brain. Bileam's mental faculties were such that he should have seen the angel. If even his ass could see the angel, his own lack of awareness could only be due to a sinful outlook on his part. Man is duty-bound to always be on guard not to be the cause of something sinful.

ויקצוף משה על פקודי החיל . "Moses was angry at the commanders of the army" (31,14). How could Moses be angry at these commanders seeing he had not commanded them to kill the women? There was no need to issue such a command since common sense should have told the commanders that the women who had been the chief seducers were to be the targets of that expedition. We find that even a Bileam acknowledges guilt when confronted by an angel whose presence he had not been aware of, and he says: חטאתי כי לא ידעתי, "I have sinned though I was unaware" (Numbers 22,34). Failure to use one's common sense is culpable. I have elaborated on this in my commentary on the last chapter of tractate Sanhedrin.