In 1x01 Will smiled after saying that - he finds it grotesquely humorous because it seems absolutely ridiculous. Even though he knows it's happening, it's not personally attached him. In 2x04 Will looks pained because this revelation, "Hannibal the Cannibal", hurts him, affects him personally. He cared about Hannibal. He believed in Hannibal. He trusted Hannibal. Now that's all been torn away from him and he still wants to be in denial.