There are many synonyms for the phrase "dastardly killing" that vary in intensity and connotation. For example, "murderous act" implies premeditation and deliberate intent to kill, whereas "brutal slaying" suggests a violent, savage act. "Assassination" generally refers to the targeted killing of an important person, while "executing" implies an official or legal act of killing. "Manslaughter" indicates a killing without intent, and "homicide" is a broad term encompassing any killing of a human being. Each synonym carries its own nuances and implications, emphasizing different aspects of the act of killing.