Buddha said many times killing animals in rituals like the Brahmans did, they were also paid for this..... wouldn't help anyone attain a better place in their next life. I think that is where the "do not kill" came from. Plus he also told stories about killing needlessly and do not murder....Buddha came from the Warrior class and I love his life story as well!
If you read some of the comments on this post the reaction is about the title and the pictures, not the actual teaching. What is worse than eating meat? Reacting with ignorance, hatred, greed, to what you like or dislike! The comment section here is a teaching in itself!
Excellent thoughtful reply, thank you @jaredcordon.