It is not really a hard rule, I think there are a lot of situations where you have to hurt someone such as self-defense, having to eat other animals to survive and such. So it’s like a soft rule of thumb, as there are a lot of situations where hurting someone is justified.

And concerning killing your own species versus other animals, I think we naturally tend to have more empathy for other people and especially the ones closer to us. Also killing a non-human animal outside of self-defense can be justified by needing food, but, well, eating another human seems to be worse than eating a cow.