Assassination of Carter Harrison III
On October 28, 1893, Patrick Eugene Prendergast fatally shot Carter Harrison III (the mayor of Chicago) inside Harrison's residence. Prendergast's assassination of Harrison was driven by a delusion Prendergast held that he was entitled to be appointed the city's corporation counsel (a role he held no qualification for), and that Harrison had wrongfully deprived him of this.
Source: Wikipedia — Assassination of Carter Harrison III (CC BY-SA 4.0)