There is a difference of opinion concerning the repentance of a murderer. The majority of the scholars say that there is a chance for complete repentance. They base their conclusion on general verses like the following:
"Say: "O 'Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allah, verily Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful." (Soorah az-Zumar, 39:53)
"Verily, Allah forgives not that partners should be set up with him in worship, but He forgives except that (anything else) to whom He pleases, and whoever sets up partners with Allah in worship, he has indeed invented a tremendous sin." (Soorah an-Nisaa', 4:48)
Furthermore, there is a hadeeth recoreded by Imam al-Bukhari رحمه الله and Imam Muslim رحمه الله which states that a man killed one hundred people and Allaah still accepted his repentance. They also use the following hadeeth recorded by al-Bukhari and Muslim: "Allaah laughs at two men, one who killed the other and they both enter Paradise. One fought the other for the sake of Allaah and was killed [by the other]. Allaah repented toward the other and he embraced Islam and became a martyr." (However, this hadeeth is irrelevant because the embracing of Islam wipes away all previous deeds, including associating partners with Allaah سبحانه و تعالى. One cannot argue based on that hadeeth that if a Muslim murders another Muslim that he can repent.)
Another group of scholars, including Imam Abu Hanifah رحمه الله and Imam ash-Shafi'i رحمه الله, say that the murderer is under the decision of Allaah سبحانه و تعالى. He may forgive him or punish him, with or without repentance.
A third group of scholars say that there is no repentance for the murderer. This is the view of Sayyiduna Ibn 'Abbaas رضي الله عنهما, It has also been narrated from Sayyiduna 'Abdullaah Ibn 'Umar رضي الله عنهما, Sayyiduna Abu Hurayra رضي الله عنه and other early scholars. There is also one narration from Imam Ahmad رحمه الله to this effect. The evidence for this view are the following:
A hadeeth states, "It can be expected that Allaah may forgive every sin except [for the sin of] a man who intentionally kills a believer or a man who dies as a disbeliever." 
There is also another hadeeth with a similar implication. It states, "Allaah refuses to make for the killer of a believer any form of repentance." 
Yet another moving hadeeth related to this topic is the following: Sayyiduna Ibn 'Abbaas was asked about a person who murdered a believer and then repented and believed and did good deeds and was then guided. Ibn 'Abbaas said, "What repentance is there for him? I heard your Prophet صلى الله عليه وسلم say, 'The killed person will come hanging on to the killer with his veins flowing with blood. He will be saying, "Lord, ask this person why he killed me."'" Then he [Ibn 'Abbaas رضي الله عنهما] said, "By Allaah, Allaah revealed that [that is, ayah 93 of Soorah an-Nisaa (3)] and He did not abrogate it." 
Based on another narration, some argue that Sayyiduna Ibn 'Abbaas رضي الله عنهما made this statement with respect to someone who was intending to commit murder. Hence, Sayyiduna Ibn 'Abbaas رضي الله عنهما told him that there is no repentance for the murderer in the hopes that it would sway him from committing murder. Although it is difficult to accept that interpretation (unless the narration can be shown to be authentic), the hadeeth of the Messenger of Allaah صلى الله عليه وسلم seem to be clear on this point. All of the arguments of those who say that there is repentance are general arguments. They cannot overrule the specific arguments found in the hadeeth mentioned in the text above, all of which have been graded acceptable by Shaykh al-Albani رحمه الله.
The strongest conclusion, in this author's opinion, and Allaah سبحانه و تعالى knows best, is that when a murderer repents, that repentance will be of some benefit to him. However, that repentance is not enough to completely wipe away the heinous sin that he has committed. Therefore, he will still have to face some punishment in the Hereafter for his sin. [For some of the above information and for arguments by one who concludes that there is a complete repentance for the murderer, see Haamid al-Muslih, Al-Maasi wa Athaaruhu ala al-Fard wa al-Mujtama (Jeddah: Maktabah al-Dhiyaa, 1992), pp. 337-342] 
 Recorded by Ahmad, an-Nisaa'i and Abu Dawood. Hasan according to Shaykh al-Albani رحمه الله. See, Silsilat al-Ahaadeeth as-Saheeha, vol. 2, p. 25
 According to Shaykh al-Albani رحمه الله, this hadeeth was recorded by Muhammad Ibn Hamzah in his Hadeeth, al-Waahidi in al-Waseet and al-Dhiyaa in al-Mukhtaar. Shaykh al-Albani concludes that its chain is saheeh. See, Silsilat al-Ahaadeeth as-Saheeha, vol. 2, pp. 309-310
 "And whoever kills a believer intentionally, his recompense is Hell to abide therein, and the Wrath and the Curse of Allaah are upon him, and a great punishment is prepared for him." (4:93)
 Recorded by an-Nisaa'i. At-Tirmidhi has something very similar. Shaykh al-Albani رحمه الله has declared a hadeeth with similar wording to it saheeh. See, Saheeh al-Jaami', vol. 2, p. 1333. This hadeeth may shed light on the previously mentioned point that there is no repentance for the murderer of a believer. If a person wrongs another, part of his repentance is to get the other person's forgiveness. If the other person is dead, he cannot ask him for forgiveness. On the Day of Judgement also, the murdered person will come hanging on to the murderer, as mentioned in this hadeeth. However, and Allaah knows best, a person's sincere repentance from murder may reduce a person's punishment in the Hell-fire.
 Commentary on the Forty Hadeeth of Imaam an-Nawawi رحمه الله, vol. 2, pp. 1064-1065
Tawbah (Repentance) - http://www.muftisays.com/forums/53-duaas--supplications/5476-tawbah-repentance.html
Forgive me for I have sinned - http://www.muftisays.com/forums/du-aas-supplications/5116/forgive-me-for-i-have-sinned..html