Death

PREMATURE DEATH
How can death be premature? You die when your heart stops beating and your eyes close for good...end of story. The only thing I can tie this logic to is a bases on a specific life expectancy. If you expect to live to say...100, then actually dying at 99 1/2 would be premature.

CERTAIN DEATH
As oppose to possible death? Sorry, you don't have a choice in the matter. One thing about life you can bet on is that you will die. All death is certain.
This is a phrase best used in context. For example...if you are consulting a doctor about an illness, possible death would sound better than certain death. However I don't believe a doctor should EVER take away ones hope of recovery...no matter how grave.