Please answer the question about the plot logic of the crime without evidence

The Crime Without Evidence was a mystery novel. It mainly told the story of a policeman who was fired because of poor performance and became a private detective to investigate a series of cases to reveal the truth of the crime in the entire city. Here are some questions about the logic of the story: Why did the protagonist give up his identity as a policeman and become a private detective? There were two ways to answer this question. On the one hand, he had encountered many difficulties in his career, including poor performance, distrust of his leaders, and so on. This caused him to feel disappointed and tired of his career. On the other hand, the police encountered many restrictions and obstacles when investigating the case. He could not use his police identity to investigate the case, so he needed to use the identity of a private detective. Why did the protagonist, the police, find evidence to prove the truth of the crime in the entire city? There were two ways to answer this question. On the one hand, the protagonist, the police, became a private detective and accumulated a lot of experience and knowledge through investigating many cases. This knowledge and skills helped him find some evidence and clues. On the other hand, the protagonist, the police, had keen observation and investigative abilities. Through his own experience and knowledge, he found some useful clues. Why did the protagonist police discover the existence of another case while investigating one case? There were two ways to answer this question. On the one hand, the protagonist, the police, found some evidence and clues while investigating a case, which may help him investigate another case. On the other hand, the protagonist, the police, might have been inspired by another case and discovered the existence of the first case. Why would the protagonist police choose not to expose the murderer of a case when investigating it? There were two ways to answer this question. On the one hand, the protagonist may think that the murderer who exposed the case would pose a threat to his life or he hoped to protect the public's safety by covering up the identity of the murderer. On the other hand, the protagonist, the police, might think that exposing the murderer of this case would have a negative impact on the social morality of the entire city, so he chose not to expose this case.