Because it stops not only the joy and fulfillment of the victim but the joy and fulfillment he represented for others. For the victim, his own joy and fulfillment may have peaked after a certain age, but the extent to which he contributes to the joy and fulfillment of others may remain high even into old age. One thing I notice as I write this is the fact that people are not accustomed to actually evaluating the worth of a person. An interesting moral exercise is to ask what you would do if you had to choose between killing several hundred prominent scientists or several hundred fishermen. This choice might arise in the case of an atomic bomb which can't be stopped but which can be deflected to one of two targets.
Life never ends.
The sperm is alive. The ovum is alive. The zygote is alive. For an excellent series of pictures, go to Early Human Stages. Finally, before anyone says that The Truth Tree is in favor of abortion, let me declare here that The Truth Tree is opposed to abortion.