Malcolm speaks out about Macbeth in act 4, scene 3. He calls Macbeth "treacherous," "bloody, luxurious, avaricious, false, deceitful, sudden, malicious, smacking of every sin that has a name." In act 5, he refers to Macbeth as a "dead butcher" and Lady Macbeth as "fiend-like."

Malcolm's name-calling in act 4 stands in contrast to his description of himself to MacDuff. Whereas presumably Malcolm means the things he says about Macbeth, when he says that, bad as Macbeth is "there's no bottom, none, In my voluptuousness" he's making a ruse, testing MacDuff to see if MacDuff's loyalties lay with his country or with him. The contrast with Macbeth is stark: whereas Macbeth truly is murderous and requires personal loyalty to assuage his guilt, Malcolm's "guilt" is a itself a lie, a trick used to expose MacDuff's allegiance not to Malcolm (or anyone else) but to Scotland itself.

His most memorable quotation on this subject is 'This dead butcher and his fiendlike queen.'

Malcolm says Macbeth is a butcher because he has killed the king and Macduff's family and others. He refers to Lady Macbeth as a "fiend-like queen" because she helped Macbeth in his deadly plot. She has lost her mind by the end of the play. Her guilt and Macbeth's coldness towards her has caused her to go insane.

His first comments about Macbeth are about how great a warrior he is.
Later, when talking to Donalbain after their father had been killed, Malcolm indicates that they (the Macbeths) are false, or at least, can be faking sorrow, and are not to be trusted.
Later still he names Macbeth as traitor, and like a fallen angel—worse because he was once good. In that section, he piles insults on Macbeth, saying that his list of sins and vices has no end—he's greedy, lustful, etc. He explicitly calls him "devilish."