That's why she is a warrior of Peace. And killing or not killing, I wouldn't say Batman is a champion of Peace xD
If to kill a bad guy is the only way to save other innocent people what would you do? Since you are for peace you don't do that and you let the bad guy kill whoever he wants? You wouldn't obtain Peace in that way either, you know...
She is a warrior of Peace because she does things like this:
If she can she does everything she can to seek Peace. If there's no other choice she is willingly to do things she doesn't like to do. That's much more realistic than Batman's no killing policy. Sometimes there's no other choice and you have to do what you have to do. No different from the situation a soldier or a policeman can find himself. You are in the position to kill the bad guy. If you don't do that he will kill another person. What do you do? You let him kill that person because of your no killing policy or you do it even if you really wouldn't want to do that, even if you'll have nightmares and you'll really regret it later?
Saying there's always another way is just not the true. Sometimes there's another way (and Diana would follow that other way), sometimes there's not. In fact I find that to maintain the no killing policy for some characters sometimes writers find themselves to resort to very unrealistic gimmicks.