In the natural world, killing is not wrong or right......it simply is. Is it wrong when two lions fight to the death over territory.....to be the dominant male in that territory to have the pride?
We however no longer live in the natural world. We humans have decided to strive for social structure. The only way any social structure can work is if it's citizens are not in fear of one another......they feel safe being around other members.
That doesn't make killing wrong......it merely makes it harmful to the maintanence of a society and is therefore frowned upon by that society. If lions decided to congregate in prides of millions.......without dominant males......I'm sure killing would also be frowned upon in their society.