Humans kill and train to kill as a reflex, which is very reasonable. It is not just killing to eat nor killing to express rage, but cold, calculating killing for almost any reason imaginable.
In my opinion, you couldn't make it a crime of murder to kill something or someone who inherently does not understand the rules, and makes themselves a killing threat in turn. Law is a mutual set of agreements subject to interpretations. While we can make it a crime to poach animals, for instance, we can't make it a crime for animals to kill a human being, that would be silly. We can't even put humans not of sound mind on the stand. Let alone expect animals to abide by law or human concepts like object persistance (that something exists when it's out of sight), let alone property or rights.
Anyway, if it's a choice between dolphins and humans I choose humans. I think Earth has a better chance of fighting off asteroids with humans.
(Of course you can't watch that video and not go awwwww)....