Malcolm does not trust Macduff initially. He has come from Scotland to England and left his family behind. This is curious to Malcolm, he thinks that Macduff might be a spy for Macbeth and so he says that he would make a worse king that Macbeth to test Macduff's loyalty.
Once he is convinced that Macduff is in fact on his side, he tells him that his comments about himself were not true.
"No less In truth than life; my first false
Was this upon myself. What I am truly," (Act IV, Scene III)
Sorry im in 7th grade i just finished reaing that book and im telling you im not that smart and i know that!!!!!!!!!!!!!
Malcom was doing that to find out Mucduffs reaction.