If I had to choose one or the other, I'd say its a waste before you've lived your life.. and a completion after you have.
Example: I was sad my grandmother went, but I was happy for her too.. She was in pain, had lived her days, and was basically waiting for life to take its course. It is a happy situation, because she died naturally and lived a long life. It completed it for her--no more pain, she finally got to find out if her devotion to God actually meant something or not.. Something I may not know until I join her much later in life.
Example 2: A young girl my friend knew got hit by a drunk driver on the street near where we live and died as a result. I'd say its a waste.. She had her whole life ahead of her, she was young, she had no control over the situation.. In my eyes, its a waste to see people go from situations like that. I also find diseases that take lives prematurely a waste.
Just personal opinions though.. I think death is a fact of life, and something to ponder at times, but I try to avoid dwelling on it too much. I don't find death a sad thing in and of itself, only sad if the situation like example 2 occurs, but I do find it intimidating and a bit out of my league.. Id rather focus on the things going on right now.