When the religious experts slander Jesus, Jesus turns the situation into an object lesson. The Pharisees define for us what Jesus means when he says God will judge us for every "careless word" we speak. This helps disciples test ourselves to make sure we continue to make progress in growing to be like Jesus.
When Jesus tells the crowds that John the Baptist is "the Elijah" prophesied by Malachi, he was not repairing a prisoner's reputation. Jesus sounded the trumpet call of his gospel message, "The kingdom of God is at hand. Repent, and believe the good news." Malachi's prophecy of "Elijah" coming to announce the arrival of the Christ is the tell-tale sign that God is about to bring the fire of judgment against all enemies of his kingdom. For the moment, there is opportunity to join God's side by being joined to Christ Jesus. No one knows how long that moment of opportunity will last.