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.