I believe some people categorize humans and animals in the same set. God made us in His likeness and we are to rule over animals and such. But that's difficult for some to understand.
I don't agree with crulty towards animals, but back in the day that this study refers to, sacrifice wasn't done in a cruel manner and this was what God REQUIRED for forgiveness. Plain and simple.
Because Jesus Christ made the ultimate sacriface on the cross, sacrificing animals is no longer necessary.
Farmers kill animals everyday so we'll have meat to eat. I've never seen an animal killed and wouldn't want to witness it, but if were required of me for God's forgiveness of sins, I would do it in a heart beat.
I think if those repulsed by bible day animal sacrifice would just consider Jesus, the Lamb of God, giving His innocent life so we might have a chance to eternity, they would understand a bit more.