I am specifically talking about going to war that is a mitzvah such as the wars fought while conquering Eretz Yisroel in the beginning of the Neviim and the mitzvah we have and will fulfill of destroying the entire nation of Amalek. I have not seen anything written on what the Torah perspective is for the soldier going and killing people when done as a mitzvah. (Are you supposed to be besimcha? Are you supposed to have a part of you not like what you are doing?) As always sources for ideas will be most appreciated.
See Rambam Laws of Kings and Wars chapter 7 Halacha 15: "Once a soldier enters the throes of battle, he should rely on the Hope of Israel and their Savior in times of need. He should realize that he is fighting for the sake of the oneness of God's Name. Therefore, he should place his soul in his hand and not show fright or fear.He should not worry about his wife or children. On the contrary, he should wipe their memory from his heart, removing all thoughts from his mind except the war.Anyone who begins to feel anxious and worry in the midst of battle to the point where he frightens himself violates a negative commandment, as it is written (Deuteronomy 20:3): 'Do not be faint-hearted. Do not be afraid. Do not panic and do not break ranks before them.'Furthermore, he is responsible for the blood of the entire Jewish nation. If he is not valiant, if he does not wage war with all his heart and soul, it is considered as if he shed the blood of the entire people, as ibid. 20:8 states: 'Let him go home, lest he demoralize the hearts of his brethren like his own.' Similarly, the prophetic tradition explicitly states: 'Cursed be he who does God's work deceitfully. Cursed be he who withholds his sword from blood.' Jeremiah 48:10In contrast, anyone who fights with his entire heart, without fear, with the intention of sanctifying God's name alone, can be assured that he will find no harm, nor will bad overtake him. He will be granted a proper family in Israel and gather merit for himself and his children forever. He will also merit eternal life in the world to come as I Samuel 25:28-29 states: 'God will certainly make my lord a faithful house, for my lord fights the wars of God and evil will not be found with you... and my lord's soul will be bound in a bond of life with God.'"