Allaah سبحانه و تعالى says, "Surely, those who persecute believing men and believing women, and do not repent, will face the torment of Hell, and the punishment of the blazing Fire."
Imaam Ibn Katheer رحمه الله commented, "Surely, those who persecute believing men and believing women,"
meaning, they burned them. This was the opinion of Ibn 'Abbaas رضي الله عنهما, Mujaahid رحمه الله, Qataadah رحمه الله, ad-Dahhaak رحمه الله and Ibn Abzaa رحمه الله. "And do not repent"
meaning, they do not cease from what they are doing, and do not regret what they had done before. "They will face the torment of Hell, and the punishment of the blazing Fire,"
because the recompense is according to the type of the evil deed committed. 
Imaam Ibn al-Qayyim رحمه الله said: "[Allaah سبحانه و تعالى] then informed that He has prepared a punishment of Hell and a blazing fire if they do not repent, and that if they repent after persecuting His friends and torturing them with fire, He would forgive them and not punish them. This is the ultimate in kindness and generosity... Imaam al-Hasan al-Basri رحمه الله said, 'Look at the kindness of the Lord Almighty, He invites them to repentance though they persecuted His friends and burned them with fire.'  The servant should not become despondent or give up hope in [Allaah's] forgiveness and pardon, whatever he may have done, for there is no form of enmity greater than this enmity, and no form of disbelief worse than that one who burns to death a believer who worships Allaah سبحانه و تعالى alone. In spite of that, if he repents, he will not be punished and he will be added to Allaah's [list of] friends." 
That was the case of 'Umar Ibn al-Khattaab رضي الله عنه and other Makkans who persecuted Muslims and later joined the fold of Islaam. This complete erasure of sin is based on the following prophetic principle:
Abu 'Ubaydah ibn 'Abdillaah رضي الله عنهما narrated from his father رضي الله عنه that Allaah's Messenger صلى الله عليه وسلم said,
التَّائِبُ مِنْ الذَّنْبِ كَمَنْ لَا ذَنْبَ لَهُ
"One who repents from sin is like one without sin." 
To further illustrate the power of repentance, the Prophet صلى الله عليه وسلم related a story from the past concerning a serial killer. Abu Sa'eed al-Khudri رضي الله عنه narrated that the Prophet صلى الله عليه وسلم said,
كان فيمن كان قبلكم رجل قتل تسعة وتسعين نفساً، فسأل عن أعلم أهل الأرض، فدل على راهب، فأتاه فقال: إنه قتل تسعه وتسعين نفساً، فهل له من توبة؟ فقال: لا، فقتله فكمل به مائةً، ثم سأل عن أعلم أهل الأرض، فدل على رجل عالم فقال: إنه قتل مائة نفس فهل له من توبة؟ فقال: نعم، ومن يحول بينه وبين التوبة؟ انطلق إلى أرض كذا وكذا، فإن بها أناساً يعبدون الله تعالى فاعبد الله معهم، ولا ترجع إلى أرضك فإنها أرض سوءٍ، فانطلق حتى إذا نصف الطريق أتاه الموت، فاختصمت فيه ملائكة الرحمة وملائكة العذاب. فقالت ملائكة الرحمة: جاء تائبا مقبلا بقلبه إلى الله تعالى، وقالت ملائكة العذاب: إنه لم يعمل خيرا قط، فأتاهم ملك في صورة آدمي فجعلوه بينهم- أي حكماً- فقال: قيسوا ما بين الأرضين فإلى أيتهما كان أدنى فهو له، فقاسوا فوجدوه أدنى إلى الأرض التي أراد، فقبضته ملائكة الرحمة
"There was a man from a previous era who killed ninety-nine people. When he asked who the most knowledgeable person in the world was, he was directed to a monk. He went to him and informed him that he had killed ninety-nine people, and asked him if [it was possible] for him to repent [and be forgiven]. The monk told him it was not, so he killed him also and completed a hundred murders.  Then he again asked who was the most knowledgeable person in the world, and was directed to a very learned man. He informed him that he had killed a hundred people, and asked if it were [possible] for him to repent. The scholar replied, 'Yes. Who can stand between you and repentance? Go to such and such a land, as there are people there who worship Allaah سبحانه و تعالى. Worship Allaah سبحانه و تعالى along with them, and do not go back to your own land, for it is a place of evil.'  The man set out for the land, but halfway through his journey he died.  The angels of mercy and the angels of punishment fell into a dispute over him. The angels of mercy said, 'He came repentant with his heart turned toward Allaah.'  The angels of punishment argued, '[But,] he never did a good deed.' An angel then came to them in human form and they asked him to judge between them. He said, 'Measure the distance between the two lands, and whichever he is closest to will determine his fate.' They measured and found him closer to the land he was headed for, so the angels of mercy took him.    Notes:
 Tafseer Ibn Katheer, vol. 10, p. 433
 Tafseer Ibn Katheer, vol. 10, p. 433
 Muhammad, Yusri as-Sayyid. Badaa'i at-Tafseer al-Jaami' li Tafseer al-Imaam Ibn al-Qayyim al-Jawziyyah رحمه الله, (Riyadh: Dar Ibn al-Jawzi, 1st ed., 1993/1414AH,) vol. 5, p. 172
 Recorded by Ibn Maajah. According to Imaam Ibn Hajar رحمه الله and Shaykh Saleem al-Hilaali حفظه الله, it is hasan. See Shu'ayb al-Arnaa'oot's رحمه الله footnotes to Imaam Ibn Rajab رحمه الله, Jaami' al-'Uloom wal-Hikam, (Beirut: Muassasat ar-Risaalah, 1991,) vol. 1, p. 414; Shaykh Sakeem al-Hilaali حفظه الله, Eeqaadh al-Himam al-Muntaqaa min Jaami' al-'Uloom wal-Hikam, (ad-Dammaam: Dar Ibn al-Jauzi, 1992,) p. 251.
 Not only was the monk ignorant about the religion, he was not wise enough to consider the consequences of enraging a serial killer.
 This is a very important piece of advice for one who wants to repent. It will be much easier to do so if he changes his environment. His old friends with whom he used to commit sins are likely to invite him to keep on sinning with them. Others who did not participate with him may be sceptical that he will really change. Their negativity will discourage him from persevering.
 Imaam al-Hasan رحمه الله said, "We were told that while he was dying he continued to crawl forward on his chest."
 This indicates that Allaah سبحانه و تعالى allows some angels knowledge about people's internal states, not just their outer actions. It is also apparent that the angels of punishment were not aware of his inner state, since they based their judgement on his deeds.
 This is one of the many evidences for the majority of the scholars, who consider it possible for a Muslim who deliberately murders another Muslim to have his repentance accepted. For a more detailed look at this issue, see: Repentance Of The Muslim Who Intentionally Kills Another Muslim - https://www.muftisays.com/blog/Seifeddine-M/3508_20-11-2013/repentance-of-the-muslim-who-intentionally-kills-another-muslim.html
It is also evidence for Imaam Maalik's رحمه الله position that when two parties agree to make a third person a judge between them, his decision is binding upon them. Al-Mufhim, 7:90-2.
 Recorded by Al-Bukhaari and Muslim.
 Reproduced from: Shaykh Dr Abu Ameenah Bilal Philips حفظه الله, A Commentary on Surah al-Buruj, (Birmingham, UK: Al-Hidaayah Publishing & Distribution Ltd, 2007,) pp. 58-60.
Tawbah (Repentance) - https://www.muftisays.com/forums/12-virtues/5476-tawbah-repentance.html
Seeking Forgiveness - http://www.muftisays.com/forums/du-aas-supplications/5174/seeking-forgiveness.html
Forgive Me For I Have Sinned - http://www.muftisays.com/forums/du-aas-supplications/5116/forgive-me-for-i-have-sinned..html