What Good does Prison do?

I have an idea for those that are given a life sentence. Put a tracker into them, throw them onto an island with rough seas surrounding it along with great walls. Make the island lush and inhabitable and throw some animals on it. If the people survive, they get to live there their whole lives, if they don't, well they were gonna die in prison anyways, why not just cut to the chase. This would be for extreme cases where the state may be against the death sentence but a life sentence in jail just isn't good enough (like serial killers or even genocidal maniacs).