It is normal because for a very long time we didn't have a choice. I won't argue with the ethical aspects of killing animals but from a climate standpoint it makes sense to kill and eat a lot less animals.

I chose not to eat any animal products because I feel I don't need to eat animals to live and because the alternative is better for the world in general. The alternatives seem to either not live up to the claims or not be sustainable except at a small scale.