If that someone is residing outside your territory, and if you act outside the law of the country that someone reside in (or have a formal declaration of war ag

If that someone is residing outside your territory, and if you act outside the law of the country that someone reside in (or have a formal declaration of war against that country), then it is definitely not warfare. The most fitting word I can think of that describes the action of killing the person would be 'terrorism'. Seriously - sovereignty of other countries is a thing. Learn to respect that.