Humans tend to have a natural inclination towards violence and vengeful actions when they feel wronged or threatened. It could be argued that one aspect of morality is fighting these natural urges and not acting with force against those around us. I know very few people who would argue that killing someone is acceptable, however under the right circumstances most people would not object to taking the life of an individual. Self defense for example, most people would not disagree that if fatal force is necessary to preserve ones own life, then the use of that force is acceptable in that situation. To take this a step further, many people do not see lethal force as an objectionable thing when used to defend someone else either. In fact there are many, many cases in which the use of lethal force has been desired as a response to the events that took place. I think that people who are forced to kill, whether in war or self defense, live with the psychological effects of it, so while it is not something that should be punished, those people do suffer because of it anyway.