Intuitively, it is possible to understand that participating in a decision to kill is a priori wrong  While simply consulting our moral intuitions case-by-case is not always reliable (indeed many people have contradictory moral intuitions), certain moral intuitions are needed in order to morally theorize. If a moral theory was impeccably well thought out, but prescribed actions completely at odds with our moral intuitions (such as advocating indiscriminate assault and robbery), then we would rightly dismiss it out of hand. When it comes to killing, our intuition prohibiting it is foundational and widely held.