The quote is found in Act 4, scene 3. Macduff and Malcolm have been conversing and Malcolm asked him for support in their campaign to overthrow the tyrant Macbeth. Ross arrives and divulges the horrific news to Macduff, who is overwhelmed and utterly distraught. The quote which summarises his attitude to Macbeth reads:
... front to front
Bring thou this fiend of Scotland and myself;
Within my sword's length set him; if he 'scape,
Heaven forgive him too!
By calling him a 'fiend' Macduff expresses his absolute disgust for Macbeth. Furthermore, he sees Macbeth as so vile that it is his duty to rid Scotland of him and he wishes to face him, man to man - within his sword's length. Macduff is suggesting that should Macbeth escape, Heaven (God) should forgive him for he, Macduff, would not be able to do so.
It is clear from the passion in Macduff's words that he despises Macbeth. He is not afraid to face him and execute the ultimate penalty on him for his evil. It is therefore fitting that Macduff is eventually the one who faces Macbeth and slays him.