Killing a living thing is when someone or something ends that life and makes the living thing die. It means causing a death. When a human being kills another human being, it is called murder or homicide, such as manslaughter.
When a soldier kills another in war, it is called "combat". When the state kills a convict sentenced to capital punishment, it is called execution. When someone kills a powerful person it is called assassination. When a person who wants to die kills himself it is suicide, or euthanasia if killed by another. When people kill other people to eat them, it is called cannibalism.