The word "murtherous" is typically used to describe actions or behaviors that are violent, deadly or murderous. Some synonyms for this word include terms like "homicidal," "lethal," "deadly," "dangerous," and "killer." Depending on the context in which the word is being used, other possible synonyms could include phrases like "bloodthirsty," "violent," "fatal," "murderous," or "deleterious." These words all convey a sense of danger or deadly intention, and they can be used in a variety of situations to describe anything from a person's behavior to the effects of a particular substance or situation. Choosing the best synonym for "murtherous" will generally depend on the context of the text and the intended meaning that the writer wishes to convey to the reader.