Macbeth is all about misinterpretation, misunderstanding and most definitely misinformation. A 'fiend' has devilish and evil qualities and thus the mantra "fair is foul" is reinforced from the beginninhg to the end of Macbeth.
It is fitting that these words of Malcolm's should appear at the end of Macbeth (V.viii.69)when it appears that Malcolm will take his rightful place as king. The description of Macbeth as a 'butcher' demeans Macbeth's character sufficiently to reduce him to nothing more than a tradesman within the realm. Lady Macbeth's description as a fiend is apt because she is an opportunist and attempts to further her cause - to crown Macbeth - through evil and immoral and downright wicked ways.
Macbeth himself, earlier in scene viii refers to the 'juggling fiends no more believed" as he only now realizes that he has been manipulated by the weird sisters and their misleading prophesies. He was equally deceived by Lady Macbeth and her desire to be the wife of the king and to make a 'man'out of her husband - thereby making the name' fiend' even more appropriate.
A butcher feels no emotion towards the animals he kills. He sees his actions as necessary in the performance of his tasks. Macbeth continued his killing spree with no emotion, ensuring the deaths of anyone who might prevent him from performing his 'job' - even his one-time friend, Banquo. Macbeth's guilt may have afffected him but only from a fear of being caught or not attaining his 'rightful' place and only temporarily.
Macduff also calls Macbeth a fiend
Bring thou this fiend of Scotland and myself
and - as Lady Macbeth is almost an extension of Macbeth,- it seems appropriate that they both fit this description.
Malcolm has only his country and his people in mind when he ends Macbeth with words of encouragement and
..his plans to inaugurate a new era in Scotland, rewarding the soldiers, calling home exiles, and serving by the grace of God.
This is in sharp contrast to the beginning of the play when the witches and their scheming were the main focus. He is assuring the people that those who " by self and violent hands" would have brought ruin to Scotland, shall have no more effect on events. The weird sisters, Macbeth and Lady Macbeth - all fiends!