Macbeth is a tragic hero. At the beginning of the play we encounter a man who has been unstoppable in battle. We hear how he rescued Malcolm. In other words he is hero but due to several circumstances, by the end of the play he is not only a villain, he is a tyrant.
First, there is his own ambition which he tells us about. Secondly there are the predictions of the weird sisters. Finally there is Lady Macbeth who feels that her husband would be a better king than either Duncan or Malcolm. When she hears that the king will be coming to spend the night, she sees this as their opportunity. None of these circumstances alone would prompt him to kill Duncan but combined together, they prove too much.
Macbeth, as the tragic hero is flawed, and this flaw takes him from hero to villain/tyrant. Although he knows that if he kills Duncan and usurps the throne, he will die a violent death, he can't resist the temptation.
It is said that power corrupts and absolute power corrupts absolutely. Macbeth is a good example of that statement.