Macbeth is a tragedy, a drama about a high-ranking individual who has many noble qualities, but who suffers from a tragic flaw. Because of this...

It is true that Macbeth is manipulated, most notably by his own wife and the Weird Sisters, but if he did not already possess the flaws that compel him to make unscrupulous choices then his story would not have ended in his own destruction.  He, ultimately, is responsible for his own tragic ened.

Macbeth admits that he suffers from a "Vaulting ambition" but believes that it is not enough to convince him to murder Duncan (1.7.27).  However, when his wife wounds his pride, insisting that "When [he] durst [commit the murder], then [he was] a man" (1.7.56).  In other words, unless he is willing to murder Duncan, then he is not really a man in her eyes.  She calls him a "coward" and "green and pale," painfully wounding his ego again and again (1.7.47, 41).  She knows just what buttons to push.  It is only after she insults his pride that Macbeth is willing to move forward with their plan.  If he were less prideful, she would not be able to manipulate him in this way.  

Further, the Weird Sisters, as early as Act 1, scene 1, admit that they are going to attempt to deceive Macbeth.  They chant, "Fair is foul, and foul is fair," meaning that they are going to make good things seem bad and bad things seem good (1.1.12).  Therefore, when they tell Macbeth that he is going to be king, it sounds really good but it will, in actuality, be quite bad for him.  It compels him to consider how to make it happen when his ambition takes hold of him.  Were he less ambitious, less easily manipulated, or even more loyal, he would not have taken the path to his own demise.