[Repentance of the intentional killer]

The answer is that if the intentional killer repents, his repentance is accepted. The proof for that is:
1. Allah, exalted is He, mentioned in Sūrat al-Furqān the warning concerning shirk, killing, and zinā, then after that He said: "Except for those who repent and believe and do righteous deeds; for such, Allah will replace their evil deeds with good ones. And Allah is Ever Forgiving, Ever Merciful." [al-Furqān:70]
2. And He, glorified is He, said: "Say, 'O My servants who have gone to excess against their own souls, do not despair of the mercy of Allah. Indeed, Allah forgives all sins; truly, He is the Ever-Forgiving, the Ever-Merciful.' And turn in repentance to your Lord…" [al-Zumar:53–54]
3. There is no disagreement that committing shirk with Allah is the greatest of the major sins, and there is no disagreement that Allah, exalted is He, forgives it by repentance. So all the more so will Allah, exalted is He, forgive by repentance what is less than shirk, such as intentional killing.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2