"why the Blood"
"Why did Jew sacrifice animal, cutting their throat and shedding their blood"
For the life of the flesh is in the blood, and I have given it for you on the altar to make atonement for your souls, for it is the blood that makes atonement by the life. -Leviticus 17:11
Indeed, under the law almost everything is purified with blood, and without the shedding of blood there is no forgiveness of sins. -Hebrews 9:22
"Who was the 1st to sacrifice an animal to please God"
God Himself.
And the LORD God made clothing from animal skins for Adam and his wife. -Genesis 3:21