Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allaah . Indeed, Allaah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful." (Surah az-Zumar 39:53)
Everybody can repent and be forgiven. The doors of repentance are always open so long as one is alive. Even if a Muslim doesn't repent from major sins before dying, the only unforgivable sin if one dies without repenting (after the message of Islaam has reached them) is major disbelief. The intentional murder of innocents is not an act of major disbelief in and of itself though it is considered a major sin. Muslims may be punished in Hell to be purified of sins but we believe all Muslims will eventually enter:
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Allaah will say: ‘The angels have interceded, the Prophets have interceded and there is no one left but the Most Merciful of those who show mercy.’ Then He will take a handful from Hell, and will bring out people who never did any good and who will have turned into charcoal. He will throw them into a river on the outskirts of Paradise that is called the River of Life, and they will emerge like seeds in the silt carried by a flood. … They will emerge like pearls with jewels around their necks, and the people of Paradise will recognize them. These are the ones ransomed by Allaah, whom Allaah admitted to Paradise with no good deed that they did or sent on ahead. Then He will say: ‘Enter Paradise, and whatever you see is yours.’ They will say: ‘Our Lord, You have given us what You have never given to anyone else in creation.’ He will say: ‘You will have something better than that with Me.’ They will say, ‘O Lord, what could be better than this?’ He will say, ‘My good pleasure, for I will never be angry with you again.’” [Saheeh al Bukhaari]
In a narration that is relevant to your questions, the Prophet also told us the story about the man who killed 99 people (which became 100 in his quest for repentance):
“Among those who came before you was a man who killed 99 people. He then asked to be guided to the most prolific worshipper from the inhabitants of the earth, and he was directed to a monk. He went to him and told him that he had killed 99 people, and he asked whether it was possible for him to repent. The monk said, ‘No.’ The man killed him, thus making him the 100th victim.
He then asked to be directed to the most knowledgeable of the Earth’s inhabitants, and he was guided to a scholar. He went to him and told him that he had killed 100 people, and he asked whether it was possible for him to repent. The scholar said, ‘Yes, and who will stand between you and repentance. Go to such and such land, for in it dwell a people who worship Allaah, so go and worship Allaah with them. And do not return to your land, for it is indeed a land of evil.’
He left, and when he reached the halfway point of his journey, he died. The angels of Mercy and the angels of Punishment disputed with one another [in regard to his case]. The angels of Mercy said, ‘He came to us repentant, advancing with his heart towards Allaah.’ The angels of Punishment said, ‘Indeed, he never performed any good deeds.’ Then an angel came in the form of a human being, and both groups of angels asked him to be the judge between them. He said, ‘Measure the distance between the two lands. Whichever land he is closer to is the land that he is closer to [in terms of being of its people]. They then measured the distance and found that he was closer to the land that he was heading towards, and so it was the angels of Mercy who then took his soul.” [Saheeh al-Bukhaari and Muslim]
We're told in another narration that it is Allaah who commanded the land the man was leaving from to move away and the land he was going toward to come closer and then the angels measured the distance between the man and the two lands.
