I'm not sure I do disagree with the statement that Macbeth in William Shakespeare's play is a "tool", although being a professor, I would probably use more formal language to describe him.
If you wanted to write a sort of defense for Macbeth for an assignment, you might emphasize the opening of the play, where he is portrayed as a genuine hero, helping King Duncan suppress a rebellion by the traitorous Thane of Cawdor. In this part of the play, the Sergeant states:
For brave Macbeth--well he deserves that name--
Disdaining fortune, with his brandish'd steel, ...
Like valour's minion carved out his passage
This passage shows Macbeth as a brave and valiant warrior.
Next, you might argue that it is the three evil witches who tempt Macbeth to rebellion, stoking his ambitious, and planting in him the thoughts that will lead to his becoming a murderer and tyrant. Next, you could argue that Macbeth wavered initially on his path to murdering Duncan and probably would not have done so had not his wife, Lady Macbeth, strengthened his resolve. While this certainly justifies neither murder nor tyranny, you can argue that the decline of his character was not entirely his fault and that he did display a certain bravery even when he realized that Macduff would triumph.