If you are cruel to someone in a dream, then it shows the angry emotions that have been released. Perhaps the negative emotions have been lurking in you for a while, but not is the time to let them out, even if it happens only in your dreams. The unconscious mind of yours is no longer able to cope with the negativity. If somebody else has been cruel to you in a dream, then it means you should be more self confident and stand up for yourself.
Read more about dreaming of Cruel in other dream meanings interpretations.