Animals eat meat as they are all part of a natural circular ecosystem. If they were all vegetarian, what would happen to dead corpses?
Humans probably started eating meat in extreme conditions, as the human being doesn't need to do the job of any animal and eating meat is quite disgusting if you think about it.
Unless you believe that humans are just evolved piranhas, termites, sharks or vultures.
There's a theory that humans are not the last link in the food chain and humans have an invisible predator.