I don't think it is sin because the enemies are also in the same situation. War is not good to the world. But the unavoidable circumstances still continue. A lion kills deer to survive. God had made in that way. He knows very well how deer suffers while it is eaten by a lion. Even then it is made in that way. There is always a common fate which is always applicable to all creatures including human beings. That is, we are like other creatures except that we have an additional sense (sixth sense). This sense had changed only the living conditions of the human beings but not the fate. The fate is almost same to all creatures including human beings.
I guess you must also read this. God cannot watch each and every human being at the same time and cannot live with each and everyone to guide them. That's why He had created different kinds of spirits for various purposes. He had made the spirits to watch, guide and determine the destiny of the human beings from their daily activities, talks and thoughts too. Your past determines your present and your present determines your future. God has given freewill to all creatures including human beings. According to this, all human beings have all right to live a life as they wish. But if it happens to be bad and hurts someone, they have to face the consequences. These consequences are created by spirits. It's a natural system. After death no one lives in any form. Heaven and hell are only for spirits. Spirits are separate elements. A human being is not a single spirit. A human being during his/her life time is living with many spirits which have joined one by one since birth. They are knowledge, skills, feelings, emotions, interests and everything. Even thoughts are not your own. For example, when you want to take a decision on a subject, one after another the spirits think and you just listen, choose or reject the ideas which they transmit to your mind through your brain in the form of thoughts. Brain is just a media to connect the spirits to your mind. A mind is just a computer's mind. After the destruction of a computer completely you will not get it's mind. The same is the case with the human beings. Soul is nothing but an energy needed for the functionality of a body. It is not a spirit or anything else. A human being is just a robot made of flesh and bones and a toy of the spirits for their games.