My point is, that this "everything's fair in love and war" attitude is without moral basis. It's not magically OK to be evil just because you are at war, and using wartime tactics when you are not officially at war is not worse than doing them during a war.
And since when did saying "I hate you, I'm going to war with you" make it OK to indiscriminately kill civilians.
ISIS has declared war on England. Does that make it OK for them to bomb the subway?