One Two Buckle My Shoe

The dentist was found with a blackened hole below his right temple. A pistol lay on the floor near his out-flung right hand. Later, one of his patients was found dead from a lethal dose of local anaesthetic. It's a clear case of murder and suicide. But why would a dentist commit a crime in the middle of a busy day of appointments? A shoe buckle holds the key to the mystery. Now, in the words of the rhyme, can Poirot pick up the sticks and lay them straight?