Giving up Hope of the Mercy of Allah is a crime in itself:
"Say: 'O my Servants who have transgressed against their souls! Despair not of the Mercy of God: for God forgives all sins (except shirk): for He is Oft-Forgiving, Most Merciful.'" (39:53)
"And it is He who accepts repentance from His servants and pardons the evil deed and knows what you do." (42:25)
1)
Hadith Qudsi:
Allah, the Almighty, has said: O Son of Adam, so long as you call upon Me and ask of Me, I shall forgive you for what you have done, and I shall not mind. O Son of Adam, were your sins to reach the clouds of the sky and were you then to ask forgiveness of Me, I would forgive you. O Son of Adam, were you to come to Me with sins as great as the earth, and were you then to face Me ascribing no partners to Me, I would bring you forgiveness nearly as great as it.
2)
Prophet (SW) has said:
A certain person had committed 99 murders. He went to a scholar and asked, is there any chance of my being forgiven? The scholar said no, you have committed too many crimes. The man killed the scholar too, but his heart was restless, so he went to another scholar and asked the same question. He was told yes, but you must leave this town of bad people and go live in the next town in the company of good people.
So the man set out to the town he was told to go to. On the way he died. A man passing by saw two angels arguing over his dead body. The Angel from Hell said, 'His body belongs to me as he had not done any good in his life.' The Angel from Heaven said, 'His body belongs to me as he had repented and was set out to be with good people.' The man who was the passer-by said, 'Let us measure the distance of his body from the town he left and the town he was going to.'
This was done. He was found to be nearer to the town he was going to. In another version, the earth was ordered by Allah to shrink and make the distance smaller, so that he was admitted to Heaven.