What I found truly horrific about this story was not the cannibalism, or indeed anything the narrator did after being left in the pit; not that it wasn't horrible, but I was expecting it and therefore steeled myself up to it. What was truly horrible is the blindness of the narrator towards the consequences of his own actions. First, nothing had happened to him that he hadn't done to others. And he's aware of that, on an intellectual level. But he does not gain even a iota of empathy. And this is amplified by his constant surprises at things he should have expected. He knows his boss likes to kill the families of those that offend him. And, as someone who was in a trusted position and betrayed that trust, he must have offended the boss more than just a small-time competitor. Yet, he never expects that his wife and child would be among the dead. It's clear that this is what led him to being in the pit in the first place - he knew that his boss was not someone that's safely crossed, yet he had what was obviously a prolonged affair with his mistress. His betrayal of his wife was not simply the infidelity, which he does recognize, but the way he put them at risk, which he never thought of.
This short-sightedness, this inability to recognize that his actions could have consequences on him just like everyone else's did, were the true horror - because it's that short-sightedness that lets people like his boss, and like himself, to gain ascendency.