I mean, if full-blown sapience is the cutoff point, then you should be allowed to kill children up to like, age 3, and not get in more trouble than you would if you killed a dog. If any form of sentience is the cutoff point, then it should be illegal to kill most animals and you should be charged with murder for setting mousetraps or for getting an abortion. These are very subjective definitions you're throwing around as if we have empirical data to support any of them... so we have to come up with some arbitrary distinctions.