Would it be right to say Macbeth is the author of his own tragedy? If yes, then why???
I agree. While outside forces clearly did influence Macbeth's decisions and actions, in the end he is the one who had to decide to act. When he succumbs to his wife's pressure, that's his choice. When he orders Macduffs family killed, that's his choice. When he orchestrates (and perhaps even participates in) Banquo's murder, that, too, is his choice.
Here's a perfect example. As Lady Macbeth is presumably off committing the murder of Duncan, Macbeth is seeing bloody daggers and bemoaning his eternal fate of damnation. Obviously his guilt is already getting the best of him. When his wife comes back and announces she could not complete the task, he has the perfect opportunity to stop the entire plan--kind of like being given a second chance. Instead, he runs right off and does the deed. No outside influence propelled him forward then. In fact, Lady Macbeth might even have understood if he simply said this is a bad idea and we shouldn't do it. None of that happens, though, and his fate is sealed.
In the end, as he's preparing to meet Macduff and discovers how the prophecy has deceived him. He says:
He understands he's been duped, but he then takes responsibility. Finally, he is able to do one last moral thing and he chooses not to start the fight with Macduff. He understands, I think, that he alone is responsible for what he has become.
Much of this is going to be contingent on whether or not one feels the outside forces on Macbeth held more of a sway on his actions than his own sense of self. In my mind, I think that there were other forces that did exercise some level of control on him. Yet, I think that he is the author of his own narrative. At any and all points, Macbeth could have exercised enough autonomy to stop or to halt his descent into immoral depths. Yet, he didn't. He became consumed with his own sense of self and became driven to appropriate the world in accordance to his own subjectivity. This compelled him to become an active participant in his deeds. Even when his wife began to show signs of regret and remourse, Macbeth keeps on progressing into a depth that should not be entered and within this, he becomes his own agent of action. In the final moments when he talks about "a brief candle," there is a sense of acknowledgement of his own depths, the sad level to which he has descended.
F. Scott Fitzgerald once said "show me a hero and I will write a tragedy." Macbeth was certainly a hero and he is the only one responsible for his actions. Yes, there are the predictions of the weird sisters and Lady Macbeth pushing him but ultimately he is responsible (as we all are) for his actions. Whereas Lady Macbeth feels that a little water will wash away the blood, Macbeth know that is not the case. "Blood will have blood, they say..." He knows that if he takes the crown by force, it will ultimately be taken from him by force.
We can all justify our actions and try to put the blame on others or say we are victims of fate or whatever but in the end we are all responible for our own lives. We make own choices. Sometimes they are not good choices. Macbeth makes some bad choices.