I have been told that in Islam execution of a person has to be only by the sword.

Answer

According to Islam, the only form of execution permissible, is execution with the sword. No other method is permissible. Even in the early days while there were other methods such as hanging, only the sword was used. The Fuqaha (Jurists of Islam) have ruled that only the sword is permissible. There are two benefits in this Islamic form of execution:

(1) In comparison to all other methods, it is the swiftest and the least painful. An expert executioner will sever the head from the body with one swift strike of a sharp sword. Death is therefore swiftest. In a second the head is separated from the body. The person therefore feels hardly any pain. Once the head is separated, pain impulses cannot reach the brain to register the pain. The separation of the head terminates the human being as he existed. Neither the head alone nor the headless body feels the pain. Pain will be felt only when the body is intact.

(2) In Islam, punishments are meted out in public. It has tremendous deterrent value. The gory sight instils fear in people, and criminals are sobered up. They see what is happening and become fearful. Execution in Islam must be in the public. It is not executed in a hidden prison where the deterrent value is eliminated.