In the West, most will happily slaughter a cow but never a dog (and by slaughter, I mean allow someone else to slaughter it for you, but never be OK with doing it yourself). In many Indian states, cow slaughter is a crime. Many places in China traditionally ate dog meat. People all over the world make more-or-less arbitrary decisions about which animals are OK to kill, and that's before even talking about their fellow humans.

I'm not a nihilist and I do eat meat. I think we should minimize suffering to others. But I can see how anybody could be conditioned to think otherwise. It's not some inherent human instinct to want to preserve others' lives. We've had to develop that instinct culturally.