Why does malcolm portray himself as a potential, sinful tyrant?

Malcolm is just testing Macduff. He knows that even though Macbeth is a tyrant, Macduff did betray him by coming to England. Malcolm tests Macduff to see exactly how loyal he would be to him should he become king. It's all rather silly; Malcolm pretends to be this money hoarding sex maniac. Macduff says he is okay with all this up to a point. Apperently Macduff passes the test.