The Eastern Christian perspective is killing another person is always sinful, since it destroys something sacred, made in the image and likeness of God. Of course, sin has a rather different connotation in Eastern Christian theology than it has in Western theology.
In the West, sin is a violation of divine law, for which one incurs a penalty (hence the concept of penance). In the East, the word for sin is the Greek "hamartia", which means "missing the mark"--a failure to live up to the divine ideal. So, the West has had to devise a moral-ethical construct (Just War Theory) to define when killing is permissible and when it is not.