How did the Prophet (peace be upon him) curse two men when he said: "I was not sent as a curser, but rather as a mercy"?

The Prophet's (peace and blessings be upon him) statement: "I was not sent as a curser" was made after he had cursed a group of people. Then Gabriel descended and forbade him from cursing, citing the verse: {Not for you, [O Muhammad], is the decision whatsoever, whether He accepts their repentance or punishes them, for indeed, they are wrongdoers} (Quran 3:128). It is possible that the intention was to negate frequent cursing, or that those whom the Prophet (peace and blessings be upon him) cursed were exceptional cases.