[Killing a man who does not pray and does not fast]

The answer – and Allah is the One who grants success – is that it is hoped for the one who has repented and regretted killing a man who does not pray and does not fast [that his repentance will be accepted], and this is with respect to what is between him and Allah, exalted is He. The proof for that consists of several matters:
1. It is permissible for the Imām of the Muslims and their ruler to kill the one who does not pray and does not fast, after inviting him to repent.
2. It is permissible for the Imām of the Muslims to launch a raid against a village in which no adhān is heard and in which the prayer is not established.
3. His saying, exalted is He, while mentioning the polytheists: “But if they repent, establish prayer, and give zakāh, then they are your brothers in religion.” [al-Tawbah:11]
4. What has been established that the prayer is one of the pillars of Islam, and likewise fasting. So the Islam of one who neither prays nor fasts is not valid.
On the basis of what has preceded, it is therefore hoped that the repentance of the aforementioned man, who repented after killing the one mentioned in the question, will be accepted. That is because, in his killing of that man, he killed one who deserved to be killed. However, he did disobey Allah, exalted is He, in that he undertook that which does not belong to him, for killing one who does not pray and does not fast is the prerogative of those in authority, not of the subjects. From here the scholars say: there is no qiṣāṣ upon one who kills a person who deserves to be killed – such as an apostate, a muḥṣan adulterer, and so on.
Moreover, what we have mentioned is further supported by His saying, exalted is He: “And whoever kills a believer intentionally – his recompense is Hell, wherein he will abide forever; and Allah has become angry with him and has cursed him and has prepared for him a great punishment.” [al-Nisāʾ:93] For He, exalted is He, has decreed a severe threat for one who kills a believer; and the killing committed by the man mentioned in the question does not fall under the threat mentioned in the verse, for whoever does not pray and does not fast is not a believer.

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