Actually, Napoleon Bonaparte was totally defeated after his failure to regain the power in France. In fact, his defeat at Waterloo symbolizes the end of his epoch because he will never recover from this defeat. It is necessary to remember that Napoleon Bonaparte was a man who had managed to defeat many states and many monarchs but after Waterloo he was practically exterminated as a politician, commander in chief, or leader of nation. He was defeated not only physically but morally as well. Consequently, it was unnecessary to send him to St Helen and, instead he could remain on Elba.
Moreover, it was not only his personal defeat but also the defeat of the French army which, by the way, is tired from the permanent wars Napoleon led throughout the last couple of decades. This is why the defeat at Waterloo was actually the last straw that guaranteed that French people would not participate in the war because they did not want to get slaughtered anymore. In fact, this defeat perfectly showed that Napoleon was just a man who could lose the battle.
Also, it is important to realize that Napoleon grew older and he physically could not afford the struggle for the power in France anymore. Moreover, even if he could, he would be unable to keep scheming and plotting against the official rulers of France since all his supporters either betrayed him or were destroyed.
In such a way, it was absolutely unnecessary to send him to St. Helen this aging man thousand miles away from Europe converting him into a martyr, while if he remained on Elba, European rules have maintained their face and made Napoleon a subject of jokes, instead of a hero imprisoned on the edge of the world.