Because the "interior life" is demonstrated by observable behavior. It is nonsensical to speak of animals with a "secret rationality" that exists apart form their behavior. This would be a dualistic position that posits that these animals are two things, not one: one that is rational, the other irrational in expression. This doesn't satisfy the demands of parsimony. In fact, it isn't even coherent.

The hallmark of rationality is language, and I mean language in the full sense, not merely signaling or expression of emotional state. The descriptive and argumentative functions are what are characteristic of rationality.

> Why do you think personhood is even required for murder?

Because, as I wrote, personhood is composed of rationality - the ability to comprehend reality - and the ability to make decisions freely among rationally comprehended alternatives. If you can't understand reality, then it is nonsensical to speak of having the ability to choose freely - that's why contracts signed by mentally incompetent people are void, because rational comprehension was a missing element and thus free consent. Without the ability to choose freely, we have no culpability for our actions. We had no choice in the matter! So, a non-person can only kill, but never murder.

> Does your pet have enough of whatever makes personhood important to qualify? [etc, etc]

I don't understand what your getting at. My pet is not a person, because my pet does not rationally comprehend reality. Comprehension is not mere sensation. Animals absolutely perceive the world. They have emotions. But rational comprehension is more than sense perception and brute imagination. It is abstraction, which is to say, the formation of intensional signs - universal concepts - like "human" and "mortal" from the sense experience of particular instances which allows me to form propositions like "every human is mortal" and from there inferences like "every human is mortal / Socrates is a human / therefore, Socrates is mortal".

No other animal that we know of demonstrates these capacities, and therefore, no other animal is personal.

Thus, murder is a species of homicide. The specific differences of murder relative to homicide is that it is voluntary, premeditated, and malicious.

The law merely recognizes this distinction. It doesn't construct some convention around homicide. Indeed, law in general is a particular determination of general moral principles within a particular jurisdiction.

So, a lion doesn't commit murder, because a lion's actions are involuntary and neither malicious nor premeditated. Also, while a lion can kill a person or non-person, it is not capable of homicide, because its meaning specifically pertains to the killing of one person by another.