Xiao Yi paused briefly, and then Xu Jing continued: "When we investigated the case of Zheng Shouyan before, I told you about this type of serial killer. It's a murderer who commits multiple crimes and kills several victims in a series, and they often have certain patterns and preferences in their actions which are not easily changed, especially regarding their killing frequency and methods.
If there are changes in their crime patterns, it is highly likely related to a personal lifestyle change or a psychological transformation, such as Zheng Shouyan suddenly speeding up his killings because he discovered he had diabetes and realized he didn't have long to live, which unleashed his suppressed murderous desires."
Although Xu Jing was a forensic doctor, she often collaborated with the police in investigations. The department she often worked with had an outstanding criminal psychologist, and Xu Jing, exposed to their discussions, learned something about psychological crimes.
