Malcolm pretends to be even more evil than Macbeth. Why does he do this? What does he discover by doing so?

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.