Why does lady macduff's son say liars and swearers are fools?

Lady Macduff's son say liars and swearers are fools because in the story he mentions that since there are more of swearers and liars than honest people, they should get together and hang the honest folks. Moreover, he thinks that it would not be sensible to leave themselves in jeopardy when they are already able to overcome the honest people.