If a God existed and told me to kill my child, I hope I would resist the urge to obey. It would be difficult for that God to prove to me it was perfect and God in light of the (to me) clear evidence otherwise. If a very powerful alien appeared, did some cool magic/technology demonstrations, claimed to be "God" and told you to kill the person next to you, would you do it? I'd hate to kill someone because some juiced up teen alien was pranking the savages.

I like laws, they serve a good purpose. But I like for them to be discussed and argued over and changed as needed. The Bible has some great wisdom in it, but it doesn't bat 1.000 in that regard. Neither do our laws, but fortunately we don't claim they are perfect and should never change.