Due to the perceived powers of witches and wizards, many would fear them and dare not challenge them. Hence these witches and wizards were seen as infallible with no one to limit their ways. In this way they were seen like the authoritarian chief or king who could not be challenge because of their overbearing authority. With this simple example our elders passed on the lesson that wicked people usually feel a sense of infallibility and others reciprocate this by not having the courage to challenge them.
The proverb is used to explain the feeling of infallibility that evil people seem to have and consequently why some people do little to challenge these wicked people. It is also used advise against challenging wicked people without preparation and valiance.