Despite the less than civil tone, Yum has a point. I
can't prove that it's immoral to kill a self-aware being. Of course, it's rather difficult to prove anything is immoral, since "morality" and "ethics" (I ought to have used the term "ethical", really) are abstract. One can't prove that it is immoral or unethical to murder, either, but I'd say that most people can agree on that one.
That said, I
can make an argument for the case that killing a self-aware being is immoral/unethical (I'll stick with ethics from now on). We can all accept, I assume, that (barring starvation that necessitates cannibalism) it is unethical to end the life of a human that is not threatening us and has not committed any crimes. Now we can use this assumption to compare the acts of killing other animals and argue that they are ethical or unethical.
The defining characteristics of humans are memory, social relationships, and self-awareness. (You can argue these, of course, but I'll bite back.)
Cows lack social relationships and self-awareness, and have limited memory. Chickens also lack social relationships and self-awarness, and have an even more limited memory. Dolphins have strong social relationships, memory on par with humans, and self-awareness. So, in order from most similar to least similar, we have dolphins, then cows, then chickens. Dolphins are thus comparable to humans in ethical terms while cows and chickens are not, and killing a dolphin is justifiably called "murder."
Not quite Q.E.D., but close enough.
Oh and regarding this:
So then killing an enemy soldier during an engagement in war is immoral? (He should have considered context with relation to his argument
Maybe you should have considered context. Isn't hunting for dolphin (i.e. killing for sport, since no one needs dolphin flesh to survive) a little different than killing an enemy soldier, who is, presumably, a threat to one's own life?