The word "more killer" usually refers to something or someone that is deadlier or more dangerous than what is presently known. To identify appropriate antonyms for this term, we need to focus on expressions that indicate less dangerous or that offer safety. Some suitable antonyms for "more killer" could be less lethal, less harmful, less toxic, less deadly, less dangerous, or safer. In essence, these words are contrasting because they suggest the opposite of increasing the level of threat, as such, this shift in terminology could facilitate clarity of communication when discussing contrasting topics or situations.