The reason a life is so precious and not to be considered lightly is that the incarnated soul has taken a great effort and thought to be in the material body. If a life is lost too soon, it puts all the effort to naught and the soul has to do it all over again.
Not all lives are pre-fated.
Sometimes accidents happen and the spirit disembodies itself too early. It has not learned the full lesson and has to incarnate once more. This would be in the case of free will suicide or self-abuse.
Therefore all life is precious and one cannot dismiss the taking of it or the loosing of it lightly.
In one way, death is a joy because the spirit is released and in another way, it is a grief because a further incarnation must take place. Souls incarnate under great stress. The birth is the thing to mourn – not death.