Here's my theory on why the baby should not be killed.
The Demon of Lies needs a blood sacrifice of the baby by the Divine One in order to complete the summoning of the Lord of Chaos.
Since the baby was not killed, the world (and the baby) is saved, and the Lord of Chaos is still stuck in hell.
Evidence:
1. Janus enters the Council of 7 immediately after the blessing is concluded. Coincidence? He could have killed the Marked One and the Council before that, since he apparently knows of the Council's activities.
2. While the Divine One is still "dead" from the blessing, several months have passed without any summoning. The moment the Divine One returns, the summoning for the Lord of Chaos starts.
3. The Lord of Lies allowed the Black Ring members to deal with the Divine One one at a time, instead of letting them all gang up to kill the Divine One. Reason is he wants the Divine One to live (but still carry on the ruse).
4. If the Lord of Chaos has taken over the baby's body, he should be able to very soon use his power to take over the world. Would the Lord of Chaos be so stupid as to leave himself vulnerable for 20 years until he grows up and comes to power? For that matter, why not sacrifice an innocent(?) adult to save all the trouble.
5. If the Black Ring can kill 2 marked ones that easily, how is it that the last remaining marked one is so hard to kill? Reason is they purposely let the last marked one survive.
So it was all an elaborate trap to make the Divine One summon the Lord of Chaos. Luckily the Divine One is smarter.
