Death is simply the natural ceasing of life. Without death everybody would be miserable. How would you like it if you were eaten, and retained conciousness whilst being digested? Not exactly a very pleasant scenario. If nothing died, we'd be up to our knees in moaning people. Death is for the best: Without it everyone's Quality of Life goes way down, people feel gnawing hunger of starvation due to being forgotten but never having the pain alleviate, there becomes a true crisis for food, etc.
Basically, without death, we can't survive. And with it we can't either. I'd prefer death to exist than not.