When you dream killing means: According to the context, killing in dreams can have a negative or positive connotation. Dreams that kill are a way for the dreamer to end or eliminate aspects from the past that do not serve any purpose. This vivid and graphic style can be used when drastic or unanticipated changes occur. However, the nightmare of death is often associated with someone or something trying to kill the dreamer. Dreaming of killing a yellow serpent can signify a lack of jealousy. Negative symbols like rat, mouse or snake can be killed to remove any unconscious traits or aspects that may have been a threat.
(in Dream Dictionary)