Are people allowed to make mistakes? A parent might make a mistake with a child for example not letting them go outside for a couple hours ,but that doesn't make them a failure. It's easy to make mistakes, people do it all the time. Atticus has made some mistakes in TKM ,but that doesn't make him a failure. The first reason why he isn't a failure is because he is a single father trying to raise two kids and is a lawyer at the same time. He has an extremely difficult job. He lost his wife to a sudden heart attack. When his kids ask questions about situations that even adults can quite understand its difficult to tell them the truth. He is trying the best he can to raise his kids correctly while trying to do a job.
