"Dream reflects your state of mind, bad dream means that your mind needs more meditation. Living beings are not Buddha, so it is natural to have bad dreams for most people. It is negative karma in the sense that it reflects your state of mind. Be relax and meditate on Om Mani Padme Hum, mantra, reading sutra and chanting of buddha name or sitting meditation.
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Negative Karma is created from a "thought" then the "action of carrying out the thought" and "rejoice especially if harm and hurt or any form of immorality is created" and all the three processes are done without remorse nor regret but for self satisfaction.
As such I and inspired to read the comment from a contributor which explains clearly why we dream in that manner and the way we can ''cure'' our disturbances of the dreams.