Separation of power us a method to prevent concentration of power in single hand. One who is vested with extreme powers is bound to misuse it and exploit it to his own benefits. Power gives threesome an autonomy and if the power us absolutely vested in one person then corruption is bound to happen given the human nature. Power does corrupt a person, he tends to use it for his own benefit without thinking about others but absolute powers leaves the person unleashed and highly prone to get absolutely corrupted. When a person gets all the power he is the boss and he has the sole discretion in the matter of decision making and going things according to his own whims and fancies. He can hardly be controlled or tamed by anybody else. He starts to think that he can do anything and this gives him the courage if getting corrupted.
Thus, when you are the boss, no one tells you that you are wrong so you start to think that you can do anything.