You can intone a chant of "progress" and "science" all you want, but if you cut the tie that binds people to good behavior, their interest will be in taking advantage of their new found freedom from morality to avail themselves of your heretofore property.
Actually that is NOT how humans work.
Some knowledge of good and bad is in our genes thanks to millenia or natural selection. Those that would harm the pack by misbehaviour were sorted out.
Human behaviour is a lot more complex than you may think. If you remove the alpha dog, there will be a period of reorganization that can be troublesome but in the end it will return back to law and order.
Why is it bad?
Because it requires killing other humans which is bad and which most humans know and understand is bad. The morality however overrides the ethics in their case (their society says that it is OK).
Eating humans is also bad due to the transmission of certain deseases, like the Kreuzfeld Jakob. So two reasons.
Kanibalism has been outruled by most societies on the planet equally and independently of each other and not just by Christians!
You do understand that societies before Christianity and Judaism had already developed a strong sense of right and wrong, based on the genetically inherited ethics of good and bad.
You do not need religion for that. Religion is just a way to explain why it is so. Also about half of the 10 commandments make no sense whatsoever in regards to morality and otherwise.