We think that we are the only creatures that matters. This can certainly be changed. We can stop our selfish and self-destructive ways by realizing that every living creature has a right to live and should not be killed for selfish reasons, this is compassion. The thinking man must oppose all cruel customs no matter how deeply rooted in tradition and surrounded by a halo. It's just as right to kill animals for food as it is to kill vegetables for food.
Humans have to kill to live, that is a fact.
This is because we can live without eating meat, but we can't live without vegetables and other non-meat foods. And since we are not meant to eat meat, it would be selfish for us to take a life to do so. Vegetables feel less pain then animals do, this is judged by their nervous system which is less developed than an animal's nervous system. Also not much of a vegetarian's food requires killing. Killing is fine as long as it's done in moderation and that we only kill what is meant for us.