He admitted to wrapping himself up in his studies, so that Antonio, in taking on the Duke's role, actually began to believe he was the rightful duke and that Prospero was incapable of ruling. Prospero seemed unaware of the idea that power can corrupt, as it did with Antonio. His brotherly love and trust blinded him to his brother's faults and actions until it was too late. He was complacent in his belief that his deputy and his team would always be there for him. But he did nothing to keep their loyalty or to lead that team.
Antonio was greedy, ambitious and ruthless, using the King of Naples, Prospero's enemy, to help throw his brother and niece into exile. He loved power and looked for more. This showed on the island, when he plotted with Sebastian to kill old Gonzalo, who loved Prospero and stood in his way of taking over Naples.
In exile, Prospero changed, he was forced to examine the situation and find the resources in himself and his environment in order to survive. He taught Miranda, and Caliban the monster, but always controlled them. Ariel, his spirit helper, might be seen as his deputy, but Prospero gave the orders, checked progress and examined the results of every activity with discussion and consultation. He took charge of an alien world and managed it so as to reach the desired outcomes. He no longer took anything on trust, but set tests and conditions, and gave explanations. As in the case of Ferdinand's love for Miranda; he tested him before giving consent to a betrothal.
"I tender to thy hand. All thy vexations
Were but trials of thy love," (Act 4, Sc.1, v. 5-6)
His experiences in life had not made him bitter, he learned a great deal on the magic island away from the real world. But he wanted to be back in business, taking up his rightful role as the Duke of Milan. To do this, Prospero understood the need to show who was in charge, but at the same time, to put the past behind him and offer forgiveness. His management skills had developed to a point where he was capable of organizing everybody and everything to his satisfaction, in order to achieve the final result that he wanted, but with no hard feelings. He showed how he felt about it when he said:
"Yet with my nobler reason 'gainst my fury
Do I take part: the rarer action is
In virtue than in vengeance;" (Act 5, Sc.1, v.25-28)
In the end, Prospero had power as himself, without spirits or magic, having kept his word and set Ariel free. By forgiving his enemies, he showed himself to be a stronger leader than when he landed in exile. He had learned never to take anything or any person at face value, to look at people's characters and motives and to always maintain control. And finally, to move forward and work together.
Shakespeare, William. 1611. The Tempest. Penguin Shakespeare, Penguin Books
London, New York. 2005.