If someone disguises himself as you and claims that you are the impostor, are you allowed to steal your wallet from him with all your money and credit cards in it? Who is the thief? You definitely have to run away, but who are the police running after? Who is the claimant and who is the defendant?
Twins were born; the first one's mission was to prepare the world to be infused with holiness, the second one's mission was to plant the seeds of holiness into the Earth his brother had plowed. But when the first twin decided he would really prefer to have fun instead of fulfilling his destiny, the whole mission was almost aborted.
Taking over both jobs was not really what Yaakov was cut out to do, but when there are no options, no questions are asked. To fill in for Esav and to accomplish the divine destiny allotted to him, Yaakov however needed the budget that went along with the project: the selfish Esav who wanted to squander it thoughtlessly had to thus be circumvented.
From his father, Yaakov naturally inherited the self-contained service of one who carefully follows in his father's footsteps, re-digging his wells and emulating his actions. He now had to acquire his grandfather's quality of initiative, normally accorded to the eldest. When Avraham Avinu passed away, his virtue of Chessed was available to whoever would claim it. Since this responsibility wasn't one of the things that Esav was particularly fond of, Yaakov claimed it for himself, strengthening his right with a legal sale. He was now the initiator, the Kohen who draws Heavenly bounty down to the physical world. When the time came to receive their father's blessings, he rightfully donned Esav's uniform, the symbol of his duty in life.
It remains Yaakov's role to carry outboth his and his brother's jobs in this world, till Esav decides to bridle his desires and realize his destiny as his brother's helper.
Adapted from Rabbi Vali's book on Bereishis. Questions and subscriptions can be mailed to: the Yeshiva