We already have a bunch of those precedents, depending on how you look at it. You're more than free to go around killing ants. No one is going to care. You can even, depending on zoning laws, raise pigs and then slaughter them for their meat. The reason that this is just not a problem in the eyes of the law is that pigs aren't people.

If you look at it another way, we have exactly one precedent: It's generally morally OK to kill members of the !X group if and only if that group consists of agents which are not people.

ETA: I hate that I have to say this, but can people respond instead of just downvoting? I'm honestly curious as to why this particular post is controversial - or have I missed something?