Do all soldiers have to kill people?

Any soldier might have to kill. Some soldiers are more likely than others to kill in war, especially the infantry.

In the infantry, and other frontline roles like the armoured corps (tanks) and artillery (heavy guns), killing is part of the job.

In support roles (e.g. driving trucks), killing is not usually part of the job, but in war it can be.

At the same time, many soldiers have had a long army career 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.