Really, Malcolm does have to be that perfect! He is, after all, the rightful heir to the throne, and Shakespeare's play was written in a time when that was a really big deal, so to speak. Factor in, also, that Shakespeare's plays were performed before royalty and that he lived and worked at the pleasure of the English monarch. Of all Macbeth's sins, regicide was considered to be the most abhorrent since it removed from the people a ruler whose power was ordained by God. The idea that the rightful heir was above reproach would be "a given" in Shakespeare's world; it further emphasizes the belief that natural order was restored when the crown was returned to the one who had the divine right to wear it.