Why? Only because 'thou shalt not kill' thing? What's morality? A system of views commonly accepted among the society members. It can be very flexible sometimes. In some African tribes cannibalism is perfectly moral. Eutanasia is moral according to the laws of some most humane countries. Morality is an empty thing compared to necessity.
Watch this movie (click on the pic for the IMDB entry):
This movie perfectly illustrates my point. Watch it.