Malcom tests Macduff by saying he is going to be a horrible king. He says that this is a test to see if Macduff will continue to support him even if he was worse than Macbeth. If Macduff said he would then Malcolm would know him for a toady who would agree to anything. He then claims to be well-nigh perfect, and that he has never told a lie before the whopper he just told to Macduff. Yeah, right. Anyway, Macduff is prepared to go along with him because he would do anything to get rid of Macbeth.
When Malcolm tests Macduff, he does so in order to make sure that Macduff supports him no matter what. Malcolm lists all the characteristics that would make him a bad king, including lust and greed. He even states he does not want to be king. However, Macduff continues to pledge his loyalty to him.
If this scene were omitted, there are a couple things that we would lose. We would not be able to understand how worried Malcolm is of betrayal, as the killings continue to increase. We would also be unable to understand how loyal Macduff is, and whether he can be trusted against Macbeth.
