It is MacDuff who understands the motives of Macbeth before others do. After King Duncan has been killed at Inverness, Macduff is the one who announces his death to the other noblemen. It is he, also, who first suspects Macbeth of desiring the throne of Scotland, In Act I, Scene 6, outside Macbeth's castle, Macduff is asked who has slain King Duncan, and he replies somewhat ambiguously, "Those that Macbeth hath slain."
When asked if he will go to Scone to see Macbeth crowned king, Macduff replies that he is going to Fife, where his castle is located. And, when Ross says that he will go "thither," Macduff wishes him well, "Lest our old robes sit easier than our new!" (1.6), indicating his suspicion. Later, he flees to England where he meets with Malcomb.
After his family is killed, Macduff realizes that Macbeth is a soldier, a cruel, professionally murderous soldier--a natural killer. So, when he addresses his troops before going into battle, Macduff would probably urge his troops to rid Scotland of this bloody and ambitious killer, Macbeth.
He hath conspired with the powers of darkness and would kill all that opposeth him or offer threat. Just as he "carved out his passage" against Macdonwald, so, too, must we rid Scotland of this villainy of nature and its foulness...this tyranny.