They were slaughtered for their flesh, for us to consume. We hung the carcases up to let them drain, bleeding out upon the floor. They were born for a purpose: To die and by their death make us stronger.
We never stopped to consider their own intelligence, or the cost of our raising them for the purpose dying. Could the feed that we used to raise them better feed individuals who ate the beasts?
Whether they were here for our consumption or not, we killed them. We ate them. We sacrificed their flesh upon a pyre, burning with smoke of the wood. We became used to the burning sacrifices.
Eventually we ran out of land, too many people, our seed grain used to feed the cattle had to be diverted. We understood that the world was dying.
So we ate whatever ate the grain.
We ate each other.
We bred our kind to provide food.
And so we had meat.
We had grain.
The problem was solved.