This is really the defining factor in a zombie story. If the person has not actually died, they are not a zombie. A zombie is someone who has died and returned from death to feed on the living. The only exception to this are the Hatian voodoo zombies who may not be dead, simply hypnotized. In other stories like 28 Days Later where infection sets in very quickly, the person is not actually a zombie in the early stages. In cases like that they are simply infected. If they are injured to the point where they actually die, then continue on, they are considered zombies.
That’s one of the tough things about a lot of modern story telling like 28 Days Later where we just assume they are zombies because they have changed into animalistic horrors feeding on us. But without death, they aren’t true and proper zombies.
In this stage it’s finally come to it. The end has come, death has set in and the victim has succumbed to his or her injuries or infection. The body begins too cool, as heart rate and respiration fail. The brain loses all activity and as time passes, rigor mortis sets in and blood pools in the extremities.
There is no guarantee at this point that this person is actually going to come back from the dead as a zombie. They might just be a victim, but there is no time to take chances with this. At this stage it’s best to put a bullet through their head or destroy the brain some other way just to be sure. If there is time burial practices can be conducted but only AFTER the aforementioned brain destruction.