Why is it wrong to kill animals?

Not sugestive, just an open question. I mean, if animals do not have a sense of death is, why is wrong to kill them. Im not gonna say animals dont have feeling or a "soul" or something obviously they do. But apart from monkeys eliphants animals do not understand the concept of death, they do not mourn their loved ones. If a bird sees a dead bird it starts humping it because it thinks its ready to mate. Death at itself is not that bad, weve all been dead for billions of years before we were born. The only cruel things about dead is mourning and the realisation of death. Do animals understand death, and mourn ? If you kill a cow in a meadow, do the other cows miss him/her ? And if they dont, why is it wrong to kill them for food ?