There are no natural predators of deer in Britain. In order to maintain a stable and healthy population of deer, a cull of some of them is required each year. This is not random however, and a deer stalker will have carried out a population count/census to determine the age and sex profile of those to be culled. During the correct deer season, barren, genetically odd or very old animals are taken. After that, other animals are selected resulting in a balanced pyramid profile with a few healthy older animals of each sex at the top and increasing numbers of each sex down to the yearlings at the bottom.
The males with outstanding antlers are sometimes referred to as trophy animals, and as part of the cull, can be shot as part of a purchased sporting package. This income helps with the management of the deer population as a whole. If population reduction is required, more females will be culled. If a population increase is required, only a selected few will be culled.
Deer stalkers have the humane dispatch of the deer at the forefront of their minds (right behind safe shooting practice), and there are many scenarios which prevent a shot from being taken. These include no safe backstop; no clear shot; the deer doesn't stop; there are other deer behind the chosen deer; it is not an appropriate animal to cull; it is a good healthy specimen which would be worth keeping to spawn future generations of healthy deer; or it is a trophy animal which could bring in much needed funds. This means that not every stalk results in a killed deer, and this can result in a "walk with a rifle" which in itself is rewarding as lots can be learnt from the stalker in the process of stalking deer.
Injured or sick animals are given priority at any time of year, whereby an exemption in law allows a person to humanely kill any deer out of season or at night "by any reasonable means" if it is so seriously injured or so seriously diseased that to kill it would be an act of mercy..