I accept that each persons life span is set before they are born. I also accept that the question is visited on Rosh Hashanah when Hashem decides who will live and who will die. All that is fine and well, until I pick up a gun and say to myself, cant I decide to end that persons life? Isnt it obvious that whether that person will live or die is now in my hands?
Using the Tisha BAv Gemarah as a backdrop we are brought into some of philosophys deepest questions: resolving the issue of how much of life is in HASHEMs control, and how much is in mans.