Why is sentience so important? It's not a trait unique to human beings, and it's considered morally acceptable to kill certain sentient beings but not human be

Why is sentience so important? It's not a trait unique to human beings, and it's considered morally acceptable to kill certain sentient beings but not human beings? Clearly sentience is not the distinguishing factor here, something else elevates human beings above other animals. I would say sentience is not intrinsically valuable either. What makes a human being a human being? It's not just sentience. Is it our intelligence? What determines our propensity for extremely high levels of intelligence above all other animals? That would be our genes. What distinguishes human beings from chimpanzees, only a small portion of our genetic code. A sperm is not a human being, and neither is an ovum, and yet when they combine and life is created that being has human genes. Women don't give birth to chickens after all. If humans have intrinsic moral value, we need to be able to decide what separates human beings from all other animals. You could name a collection of traits, but ultimately all of them our determined by our genes. So, to me, when I'm trying to build my moral framework of when it is morally acceptable to kill a human - how do I decide? Is killing an innocent human that never did anything wrong ever acceptable? If it isn't, than what makes something human and what determines "humanity"? Why is something distinctly not human, and then suddenly is human?