Are all sins forgiven after converting to Islam?

By His Grace and Mercy, Allah has made embracing Islam a cause to erase the sins that were committed before it. When a disbeliever becomes Muslim, Allah forgives all that he did when he was a non-Muslim, and he becomes cleansed of sin.

‘Amr ibn al-‘As (may Allah be pleased with him) said:

When Allah put Islam in my heart, I came to the Prophet ﷺ (peace be upon him) and said: “Give me your right hand so that I may swear allegiance to you.” He held out his hand and I withdrew my hand. He said, “What is the matter, O ‘Amr?” I said, “I want to stipulate a condition.” He said, “What do you want to stipulate?” I said, “That I will be forgiven.” He said, “Do you not know that Islam destroys that which came before it?”

Sahih Muslim, 121

Imam Nawawi said: “Islam destroys that which came before it” means that it erases it and wipes it out.

Shaykh Ibn `Uthaymin (may Allah have mercy on him) was asked a similar question, about someone who earned money by dealing in drugs before he became Muslim. He replied: We say to this brother whom Allah has blessed with Islam after he had earned unlawful wealth: be of good cheer, for this wealth is permissible for him and there is no sin on him in it, whether he keeps it or gives it in charity, or uses it to get married, because Allah says in His Holy Book:

Say to those who have disbelieved, if they cease (from disbelief), their past will be forgiven. But if they return (thereto), then the examples of those (punished) before them have already preceded (as a warning)

Quran - 8:38

(interpretation of the meaning)

This means all that has passed, in general terms, is forgiven. But any money that was taken by force from its owner must be returned to him. But money that was earned by agreements between people, even if it is unlawful, like that which is earned through Riba (interest), or by selling drugs, etc. it is permissible for him when he becomes Muslim.

Many of the non-Muslims at the time of the Prophet Muhammad ﷺ (peace be upon him) became Muslim after they had killed Muslims, but they were not punished for what they had done.

Source: islamqa.info