The "empty space" in the patient's brain is there because he apparently lost some brain tissue. The lost tissue is replaced mostly by fluid in your lateral ventricles.
The lateral ventricles are normally filled with a small amount of fluid, and are much smaller, but they can expand in certain conditions such as loss of brain tissue (old age, trauma) or hydrocephalus (a condition caused by blockage of the flow of fluid through the brain. If it can't flow it backs up...)
As far as putting him out of his misery...that gets into some dubious legal territory. They can unplug the machines at most.