Q: Why did God slaughter His Son in order to pay for sin;
A: To pay eternal justice and praise His Eternal Grace, Christ had to become the substitute for those whom were designated for eternal salvation. These chosen people were elected by God The Father in Christ Through grace.
God is eternal. Any offense done to His Divinity must be appeased eternally and entirely! Christ also was not merely permitted to just simply die and that would be the end of it....no, God was infuriated at the revolt of Adam's sin! In order to justly meet the anger of God against evil and the spirit called Sin (the noun), it was a necessity to put it to death in the flesh of Christ! But it was also necessary that Christ be punished according to the law for every type of sin! 👉🏾Capitol punishment! Punching, smiting with rods, flogging, humiliation, kicking, spitting, piercings by thorns, glass, cat bones, nails and spearhead! Then after all of this, He Suffered the flames in the seventh region of hell to incinerate every trace of sin exclusively on the behalf of His people! God is a God of just recompense! Therefore it says, Leviticus 19:35-36 KJV
 Ye shall do no unrighteousness in judgment, in meteyard, in weight, or in measure.  Just balances, just weights, a just ephah, and a just hin, shall ye have: I am the Lord your God, which brought you out of the land of Egypt.
As I have stated in the past I state it again now...God has never forgiven any sin ! All sins that were forgiven were transferred, charged, laid upon, and imputed to Yeshua Hamashaich !! If you're not an elect, your sins remain with you eternally! You're on your own! Hell is your "Central Booking"! Take this for thought:
A lion which has not eaten anything in four days and finally eats 👉🏿🐣, will the lion be full and completely satisfied? #justiceServed #christcrucified #sinpaidInFull