Does the killing of a person who insulted the Messenger of Allah, peace and blessings be upon him, then embraced Islam, become nullified? And what are the conditions for the repentance of a Muslim who insulted the Messenger of Allah, peace and blessings be upon him, but whose case did not reach the ruler?

If an original disbeliever insults the Prophet, peace and blessings be upon him, and then embraces Islam, his conversion nullifies what preceded it, and the penalty of execution is waived for him. However, if the insulter was a Muslim and apostatized by his insult, and then repented, scholars have differed regarding whether the death penalty is waived for him. The preponderant opinion is that the death penalty is not waived for him, and he is to be executed, even though his repentance is accepted by Allah. This is for the sake of avenging the Messenger of Allah, peace and blessings be upon him. The execution here is a matter of hudud (prescribed penalties) and falls under the responsibility of the Imam. As for the insulter whose case has not been brought before the judiciary, he should conceal himself and engage extensively in repentance, seeking forgiveness, and performing righteous deeds.