Killing humans is wrong because it hinders the human progress. A society that allows killing each other will not function as well as a society that doesn't allow killing, and as such killing other hmans hurt everyone, including yourself.

The same does not apply to animals. There is no advantage to be gained from not killing them, and as such it is perfectly fine to do so.

In fact, our meat-eating ways is the very reason why we are advanced creatures, as the proteins obtained from cooked meat lead to the evolution of a larger brain.