Well, you know....
You shouldn't say something bad about them back, that just makes you look really bad, even if they KOS you for doing that. There's plenty of reasons to KOS a person, and I doubt it was just based on that. If you really think of someone who does the that....like that, well, I can see a new reason to KOS, hm? Treat people how you want to be treated. If they try to start something, just ignore them.