Any soldier might have to kill in war. Some soldiers are more likely than others to kill people, especially the infantry.
Also in other frontline roles like the armoured corps (tanks) and artillery (heavy guns), killing is part of the job at times of war.
In support roles (e.g. driving trucks), killing is not usually part of the job but it can be in war time.
At the same time, many soldiers have had a long career in the army without killing anyone.
Another thing to bear in mind is that civilians, including children, are killed in war as well. Killing civilians on purpose is a war crime, but many are killed or seriously injured in the crossfire. Many thousands of civilians were killed in Iraq and Afghanistan, including by British forces.