will the 'whys?' ever cease?
Man is a fallen being. The fallen being doesn't get to decide the terms of punishment for his own trespasses. And if the fallen being is being offered a gift of having his own trespasses wiped away he certainly don't get to decide the terms on how that gift should be able to go about being received. Yet in your justice system, you propose that God permit man, the sinner/fallen being, to assume the role of judge, jury, and executioner on his own life. That's like a murderer, already found to be guilty of the crime, going on trial telling the judge how he should go about being punished. It just don't work like that. "Woe unto him that striveth with his Maker! Let the potsherd strive with the potsherds of the earth. Shall the clay say to him that fashioneth it, What makest thou? or thy work, He hath no hands?" (Isaiah 45:9)
Plus i don't know what's the point in trying to make a case for the animals as if the taking of an animal life is on par with the taking of a human life. And as if animal sacrifices is still commanded. Animal sacrifices was only commanded to Israel. It was part of their religion.
And what you mean God allows sinners to enter Heaven with no punishment taking place? Look at the sin that's in the world we live in. The punishment is already taking place on this earth as we speak. And we gotta deal with it. It's not the ultimate punishment, but for the time being, God has allowed the consequences of various sins serve as their own punishment. This way, it allows the sinner the ability to distinguish good from evil and there's a chance the sinner will realize the impact sin has on people's lives, and it will help him realize God was right and further cause him to want to turn back to God and learn to do His will. (Acts 17:26-27)