An animal is called a בהמה because בה-מה - what you see is what you get. No potential. No significant development. No dog going to Harvard [even though I wouldn't consider that something to strive for even for one of us - but that's besides the point]. No mouse writing a book on Physics.
But a human is complete potential. Hence he is called "אדם" - from the אדמה which produces fruits. Man's task is to constantly expand his potential for future achievement. If it is just about [today the popular phrase it "it's about..." so I use it:-)] who you are now when that is where the animals are holding as well. The here and now. But with man there is a whole future.
When one kills an animal he just killed the present. When one kills a human being [G-d forbid] he kills the entire future development of this אדם.
