I don't believe there's any such thing as a sin.
I don't have any reason to believe a higher power is watching what happens on earth.
I couldn't even understand why a supposedly omnipotent being would give two shits about some unimportant bipedal hominids that think they are the center of the universe because they need affirmation of life beyond procreation and eating.
That said, there are plenty of things that I think are terrible to do to a person, simply because I would not want to be treated that way.
I actually agree with Slippy here that child abuse certainly ranks among the worst.