Final answer:
Sergeant Nelson's failure to perform a mandatory duty required by law may constitute misconduct under the Illinois Criminal Code of 2012, potentially resulting in legal consequences.
Step-by-step explanation:
According to the Illinois Criminal Code of 2012, if Sergeant Nelson failed to perform a mandatory duty as required by law, it may be considered an act of misconduct. Misconduct by a public officer can be considered a breach of public trust and may be punishable under the law. The severity of the misconduct and the applicable penalties can vary, depending on the nature of the duty that was neglected and the circumstances surrounding the failure to act.