Why is killing a human worse than killing a chicken?

I hope I don’t get flagged by the FBI for this, but I’ve been thinking about it lately. Life is life, humans are animals. Why is it acceptable to decide some life is more valuable than other life? Admittedly I eat meat and don’t kill people. But I have zero problems killing the person that harms my family or others without cause. I would feel worse killing a bear in the woods that never did anything wrong than I would killing a rapist. So why is this? Why do we think animals are less worthy of life than the human animal?