If there are going to be people out there making war on other people, don't you think it's a good idea for some of those people to at least follow a code of ethics? Not 'honor' but something you can pin down and be sure of, something with the same rules for everybody.
Each organism, no matter how simple or complex, has around it a sacred bubble of space, a bit of mobile territoriality which only a few other organisms are allowed to penetrate and then only for short periods of time.