What does vladek feel guilty for

It is 1939, and Vladek has been sent to the German front. He sees a tree that seems to be moving and fires towards its center. It is a German soldier using branches for cover. The German falls and holds up a hand in surrender, but Vladek continues to shoot until he is dead. Vladek is soon captured and made to carry the German dead and wounded. He walks over to the river, finds the man he killed, and carries him back to be buried.