Actually it is very simple. Religions evolve. Acquires some parts from older religions, merges or splits, rejecting part of older religion.
It helps to get new followers, converting believers of older religion to newer religion, if you combine part of older religion. After centuries, millennia, without traces of written word, you cannot distinguish what is merged and its origin, as the all written materials never really existed, were destroyed accidentally, or destroyed on purpose, to removed traces of older religion, or are hidden in the ruins of ancient buildings buried under the desert sand..
Remembering what I just said, analyze what religions were in ancient times on lands which are now known as Middle East. In some of them there were rituals of bloody sacrifices of people on the altar, tortured and/or killed, in the name of a god, to please him/her/it, appease to stop sending plagues, Earth Quakes, thunders, flooding, etc. etc.
Look at story currently known as Binding of Isaac. In Isaac's time, Jews believed in completely different religions than 2000 years ago, and different than now. It is story showing transition of religion from older one, in which people were killing other people on altars, to religion in which it is forbidden. Instead of humans, they continued to give just animals and plants as sacrifice. In Christianity it was taken even further and the slaughter of animals on altars was also stopped (due to what you just said, it was "The Last Bloody Sacrifice"). It has been replaced by less violent, less cruel, more civilized, religious rituals.