Lose all you blood and you die. Yes, people knew that. But, they didn't know why or that it was tied to breathing. (Galen may have figured that out, but that is just after the time of Jesus.)
Here is also what they knew
Ge 9:6 Whoso sheddeth man’s blood, by man shall his blood be shed: for in the image of God made he man.
Men, so jealous of their brother, wanted to kill him
Ge 37:22 And Reuben said unto them,
Shed no blood, but cast him into this pit that is in the wilderness, and lay no hand upon him; that he might rid him out of their hands, to deliver him to his father again.
They sold him into slavery instead.
Years later they thought God had brought them to account for his blood
Ge 42:22 And Reuben answered them, saying, Spake I not unto you, saying, Do not sin against the child; and ye would not hear? therefore,
behold, also his blood is required.
Ge 9:5 And surely
your blood of your lives will I require; at the hand of every beast will I require it, and at the hand of man; at the hand of every man’s brother will I require the life of man.
It was known from the beginning.
God heard a great cry, from Soddom and Gomorrah
Ge 18:20 And the LORD said, Because
the cry of Sodom and Gomorrah is great, and because their sin is very grievous;
Ge 18:21 I will go down now, and see whether they have done altogether according to
the cry of it, which is come unto me; and if not, I will know.
Ge 19:13 For we will destroy this place, because the cry of them is waxen great before the face of the LORD; and the LORD hath sent us to destroy it.
23 And Abraham drew near, and said, Wilt thou also destroy the righteous with the wicked?
Save it for 50 righteous in it?
24 Peradventure there be fifty righteous within the city: wilt thou also destroy and not spare the place for the fifty righteous that are therein?
25 That be far from thee to do after this manner, to slay the righteous with the wicked: and that the righteous should be as the wicked, that be far from thee: Shall not the Judge of all the earth do right?
Finally, Abraham gets it down to just ten
32 And he said, Oh let not the Lord be angry, and I will speak yet but this once: Peradventure
ten shall be found there. And he said, I will not destroy it for ten’s sake.
There were not even ten left in the city. Lot, his wife and two daughters escaped before God destoyed it.
So, who were so many for such a great crying to God? No righteous were left to cause such a great cry. The blood of all the righteous which had been slain were crying out from the ground unto God. Just like Able's blood did.
You may not care what scripture says, but you keep commenting on what people knew. Well, they knew
God to avenge the blood of man from the very beginning. Therefore men began to call upon him right from the beginning.
Ge 4:26 And to Seth, to him also there was born a son; and he called his name Enos:
then began men to call upon the name of the LORD.
We do not live in a vaccume before God. What we do to others can and will be heard by God. He is their God too. That is what men knew. They wrote about it.