If one came across the anti-christ as a helpless infant still suckling at it's mother's teat and knew beyond a shadow of a doubt that the infant was in fact the anti-christ would it be moral to kill him/her/it?
I don't care if God, Jesus, Buddha, Allah, Sekhmet, Quan Yin, Bob the Cosmic Wonderfish, L. Ron Hubbard, or whatever form of Divinity you worship comes and personally informs you that this child is the Anti-Christ. I don't care if you are transported to the day the Earth ends and come back with a notarized newspaper with the child's face on it, captioned, "I did it!" Some things, no matter where they come from, are just intrinsically wrong.
Killing an infant is pretty much at the top of the list.
