Cruel to be kind has to be one of the strangest of paradoxes. It is a paradox that is filled with irony. The concept of being mean to someone is doing them a favor sounds counterproductive at best. On the surface this makes no sense at all; however an effective case can be made for this concept.
Many believe that being mean is an effective form of communication, making sure they get the message. This can be quite effective, but will it have the desired effect? It just might, but possibly at a cost. Emotions overriding logic most of the time is often forgotten. Likely, if the tone is negative then the good intentions is usually irrelevant.
Conversely, kindness may not always be the best method of communication. Kindness may obscure the importance of the message. This is not to say that kindness should not be used, but there is a time and a place for it. As a tactic it should be used first, but not always.
Cruel to be kind has to be done with great care and even love. As long as the person delivering the message truly cares about the other person the message can be effective. For the most part being cruel is the opposite of kindness. However, cruelty is unfortunately, but sometimes necessary.