What should someone do who said, "Without the Most Merciful now," while angry, and whose conscience is tormenting him?

You must offer sincere repentance for what was said and resolve not to repeat the offensive word by controlling yourself when angry. If the word was uttered by someone whose mind was overcome by anger and he did not know what he was saying, then there is no sin upon him. However, if he knew and understood what he was saying, then he is accountable and his anger is not an excuse for him. The majority of jurists hold that an angry person is held accountable for his actions while in a state of anger. As long as you have repented, then "the one who repents from sin is like one who has no sin."