Loss of any person is terrible but I think the worst is when you loose your child. It's just kind of "unnatural" that they leave before the parents do. It's probably the worst thing to wake up and realize that there will never be that screaming and running around again.
As a mother or father you feel that you are the one who should protect your child and that you are the one who should bring it safe through some situations. I always feel so sorry for the parents who loose their kids through some sick criminal. For example the ones who lost their son and daughter in Germany just a few days ago.