It’s not contrary to the church’s teachings. Evidence in support of my position: Centuries of the Pope being totally fine with mass executions and murder. Also, Holy Bible. Yes, it does say “do not kill”, it also says “kill”. For example, it says to honor your mother and father, and it says that parents should murder their disobedient children by stoning. Then, it says “don’t kill.” So, it’s more like ‘don’t kill*’. But they had not invented asterisks yet. It’s almost as if different people wrote different parts of the Bible, and it’s a contradictory mess. Anyway, if you can’t follow orders, don’t join the military. Following orders is, like, the most important part.