To anyone who would say otherwise, I would like to ask who exactly do you say defined our moral obligations? Who issued the moral commands that you feel we ought to obey? Did you? Or did someone else?
If you did, why would you think they would apply to me? You didn't create me. Why would you think they would apply to all babies? You didn't create all babies either.
If someone else did, did this other person create me? Did this other person create babies?
Someone may reply that absolutely no one defined our moral obligations: moral obligations "just are". To that I would ask, says who? You? Or someone else? And what authority does this person who claims that moral obligations "just are" have in order to declare that I must obey these supposed moral commands (commands like "never kill babies") that, according to them, no one has ever issued?
Without authority, well, there simply is no authority.
For the Christian who is uncomfortable with the idea that we ought to obey God in all things, even when we may not like the command, or even when the command may cause pain (or death) to others, I would suggest reading 1 Samuel 15, Joshua 7, and Genesis 22 along with Hebrews 11:17-19 to see what God Himself has to say on the subject. Good moral behavior is simply obedience to God (Malachi 3:18; Romans 4:15, 5:13; 1 John 3:4).
Without a God who commands, there simply is no moral authority. Without a moral authority, there is no justification for suggesting that anything is morally right or morally wrong.
Here we must note that to my knowledge, God has never commanded me to kill any children. Instead, He has provided a general command for all people not to kill humans (Genesis 9:5-6; Exodus 20:13; Revelation 21:8), which is the reason we ought not do it. Nonetheless, He does require exceptions to this general rule on occasion, including those already noted above.
If you yourself have not asked for forgiveness for disobeying your Creator, the One with moral authority over you (for all have disobeyed: Romans 3:23), it is not too late. Our God will accept anyone who has turned away from their disobedience and turned to His Son, Jesus, who was executed in our place, so that we can be forgiven (James 4:8; Acts 13:38; Romans 10:9; Hebrews 10:14).