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:

readability="0.089887640449438">

"And be these juggling
fiends no more believed,
That palter with us in a double
sense;
That keep the word of promise to our
ear,
And break it to our hope. I'll not fight with
thee."

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.