I'm an atheist so help me understand the Christian reasoning from a rational perspective:

God created man and i guess he did a bad job because we, as species, decided to be very bad in his eyes.
So after killing every man, woman and baby save for Noah and his sons and son's wives...he actually decides we deserve death and torment again.
He wants to forgive us but in order to do this he needs blood. And not just any blood but the blood of his only son/aspect of himself.
So he makes an animal/human sacrifice of his son in order to forgive us for being so shitty...but it's only going to work if we believe the story after thousands of years.

So....if he wants to forgive and give a second chance, why not just do it without demanding blood and allegiance?