But isn't that a bit unfair and quick to judge?

Yes, but I will freely acknowledge that I am a judgmental person. It's a flaw I have not been able to fully correct. Funny you should mention, I'm actually in the middle of a paper on stereotypes as we speak.

Humans do a lot of things against nature. You're using a computer right now. Nothing else in nature uses computers.

A computer is a tool, albeit an extremely advanced tool. It is not unnatural. In fact, without the laws of nature, it would not function (specifically physics). We as humans have the combined capacities for extremely high level thinking and invention, and as a result we have electronics. How is this against nature?

Purchasing food, however, I would agree is against nature to some degree, but not entirely.

It can be argued that inter-species murder is natural and yet I doubt you would agree that murder is acceptable.

Agreed, murder is not acceptable except in certain situations (self defense, defense of others, etc. Situations where it is generally considered justifiable.) In this case, I'm addressing murder under the umbrella of homicide.

However, you specifically said "inter-species" murder, which I agree is natural and have no problem with. That is the circle of life, that's how the food chain works. If you get jacked by a wolf in the woods, that's not really the wolf's fault, it's a predator and you were careless.

Intra-species murder is unacceptable for humans, and just accepted as part of nature for other species. It's different for us due to our capacity for higher-level thinking and our realization that this kind of behavior is detrimental to our civilization as a whole.

This leads me to believe that, barring your example of purchasing food, an appeal to nature is perfectly acceptable. Humans are just so far advanced compared to other species that special considerations have to be taken into account.