At first, they feel nothing because they are in the warzone headspace just they're to do their job and fight for their cause. So he feels nothing when he kills the lady and the enemy soildier
After he kills them his feelings catch up to him and he starts to feel upset and sick of all the killing then he encounters an enemy sniper. who he then outsmart and kills but is curious to know the identity of the good soldier.
AFter he finds out the enemy sniper that he just killed was his brother he doesn't feel anything because he is in shock. the story then cuts off but I imagine that after the facts sink in he is very upset and struck with grief and guilt.