Malcolm declares to Macduff that he is wicked because he has excessive lust and greed and in fact has no redeeming qualities to offset these faults. He is testing Macduff's loyalty to Scotland because he does not know Macduff well and is unsure of Macduff's motives for coming to England. Is Macduff there because he seeks some personal gain, or is he interested in the good of the country? By pretending to be more evil than Macbeth, Malcolm discovers that Macduff really is more concerned about the country than he is about anything else. Therefore, Malcolm decides he will return to England with an army to overthrown Macbeth, just as Macduff had hoped.
Malcolm's father, the late King Duncan, had a fatal flaw and that is he was too trusting -- and paid for it with his life and crown. Malcolm is not so trusting -- he pretends to be full of vice to Macduff when Macduff joins him and the English army to gather forces against Macbeth. Malcolm is testing Macduff's loyalty in a way that Duncan should have tested Macbeth's. Malcolm tells Macduff that he has been with many, many women, drank, gambled, lied, swore, and all that fun stuff -- all to gauge the effect his words have on Macduff. When Macduff turns away in sadness from these words, Malcolm has his answer -- Macduff is loyal and the one to recruit to dethrone Macbeth. Malcolm is much more canny than his unfortunate father.