It's unclear where our intuitions on the subject come from or how they work, and they are heavily .... distorted ... by various beliefs and biases. OTOH, it seems unlikely that rocks are conscious and we just haven't extrapolated far enough to realize. It's also unclear whether personhood is binary or there's some kind of sliding scale. Nevertheless, it seems clear that a fly is not worth killing people over.

Even a person who has never introspected about their moral beliefs can still know that murder is wrong. They're more likely to make mistakes, but still.