If a man kills a believer intentionally, his recompense is Hell, to abide therein (For ever): And the wrath and the curse of Allah are upon him, and a dreadful penalty is prepared for him.
According to Yusuf Al Qaradawi,
Islam has made human life sacred and has safeguarded its preservation. According to its teachings, aggression against human life is the second greatest sin in the sight of Allah, second only to denial of Him. The Qur’an declares, .. If anyone kills a person for any reason other than for (the killing of) a person or for sowing corruption in the land, it will be as if he had killed the whole of mankind….(5:35 (32) )
Because the human race constitutes a single family, an offense against one of its members is in fact an offense against the whole of humanity. The crime is more serious if the slain is a believer in Allah. And for the one who kills a Believer intentionally, his recompense is Hell, to abide therein; and the wrath of Allah is upon him and His curse, and a tremendous punishment has been prepared for him. (4:93)
The Prophet (peace be on him) said, “The passing away of the world would mean less to Allah than the murder of a Muslim man.” (Reported by Muslim, al-Nisai, and al-Tirmidhi.)
He also said, “A Believer remains within the scope of his religion as long as he does not kill anyone unlawfully. (Reported by al-Bukhari.)
And again, Allah may forgive every sin except in the case of one who dies a mushrik or one who kills a Believer intentionally. (Reported by Abu Daoud, Ibn Hibban, and al-Hakim.)
On the basis of these verses and ahadith, Ibn ‘Abbas deduced that the repentance of the murder will not be accepted by Allah. For, he argued, since in the case of an offense against another individual, the wrongdoer’s repentance is not accepted unless the wrong done to others is righted or their satisfaction secured, the question is how, in the case of murder, one can restore the murdered person to life or secure his satisfaction. Others say that sincere repentance is accepted by Allah; if it erases even the greatest sin, that of shirk or ascribing partners to Allah, why not lesser sins? Allah Ta’ala says: …Those who do not invoke, together with Allah, any other deity, nor take the life which Allah has made sacred except in (the course of) justice, nor commit zina. And whoever does that shall pay the price (of his sin). The punishment shall be doubled for him on the Day of Resurrection and he will abide in it in disgrace, except for him who repents and believes and does righteous deeds. As for them, Allah will change their evil deeds to good deeds, and Allah is Forgiving, Merciful. (25:68-70)
Thus far we have quoted those texts which warn Muslims against killing or fighting fellow Muslims. But let no one get the impression that the life of a non-Muslim is not safe in a Muslim society, for Allah has declared the life of every human being to be sacred, and He has safeguarded it.
This applies as long as the non-Muslim does not fight against the Muslims; if he does so, shedding blood is permissible. However, if the non-Muslim is of a people with whom there is a treaty or if he is a dhimmi (a non-Muslim resident of an Islamic state), his life is sacred, and it is unlawful for the Muslim to attack him. In this regard the Prophet (peace be on him) has said, Anyone who kills a person from among the people with whom there is a treaty will not smell the fragrance of the Garden, although its fragrance reaches to a walking distance of forty years. (Reported by al-Bukhari and others.) “Anyone who kills a dhimmi will not smell the fragrance of the Garden.” (Reported by al-Nisai.)
Al-Qaradawi, Yusuf (1960). The Lawful and Prohibited in Islam. http://www.islamicweblibrary.com/wp-content/uploads/2019/06/TheLawfulAndTheProhibitedInIslamal-halalWalHaramFilIslamByShaykhYusufAlQardawi.pdf