What is the penalty for sin? Was God being patient and merciful by allowing proxy blood instead of the blood of the actual perpetrators? Would you rather have God kill you the instant you or anyone else sinned? He could you know and be perfectly just.
What did He do instead? The sacrificing of lambs was a precursor to THE LAMB of God, Jesus Christ. A regular lamb was only good for a year, but Jesus was the PERFECT sacrifice and only needed to do it ONCE, and not every year.