We generally don't kill animals "unnecessarily". We kill them to eat them. If we could eat them without killing them, we wouldn't kill them.

We also don't kill them "prematurely". We kill them when it's time to eat them, or else to relieve them of pain.

Also, since we're onto wolves, wolves will kill animals and not eat them (see "surplus killing" article on wikipedia). Conversely, wolves will often eat animals without killing them first. Humans at the very least kill our food before we eat it most of the time.