What is the Sharia and ethical stance on those who committed crimes against Muslims before their conversion to Islam, and who did not seek forgiveness from their victims, relying on the principle that "Islam nullifies what preceded it"?

Islam nullifies what preceded it, so a disbeliever, upon embracing Islam, is forgiven for past sins related to the rights of Allah, as Allah the Almighty says: ﴿Say to those who disbelieve, if they cease, what has passed will be forgiven them﴾. And as the Prophet, peace and blessings be upon him, said: "Islam nullifies what was before it."

As for the rights of human beings, there is a detailed explanation:

1. A disbeliever at war (Harbi) who embraces Islam: The rights of Muslims that are due from him are waived, and he is not held accountable for what he committed during the pre-Islamic era (Jahiliyyah), neither concerning the rights of Allah nor the rights of people, by scholarly consensus. 2. A disbeliever under a covenant (Mu'ahid) or a Dhimmi who embraces Islam: The rights of human beings concerning blood, wealth, and honor are not waived from him. Rather, he is demanded to fulfill them, and he must render the rights to those he wronged or seek their forgiveness.