The worst sin is Child abuse.
You can kill another person, but that person will not cease to exist. If you honestly belief in an after life, than you must believe that you can't truly take a life, but destroy their mortal vehicle to experience and interact with others with mortal bodies.
Yes killing is very bad.
But to harm a child, an innocent being is the most disgusting thing a person can do. The child will grow with the pain and psychological turmoil from that abuse and become a person that can abuse others. It creates a cycle of pain.
In my opinion, child abuse is the worst thing in the world.