What is the ruling regarding a young man who killed his niece for marrying an infidel after he invited both of them to Islam and she refused to return, and will he be punished in the hereafter given that he is religious and from a non-Muslim country?

A Muslim woman is not permitted to marry a disbeliever, based on the Almighty's saying: ﴿And do not marry disbelieving men [your women] until they believe﴾ [Al-Baqarah: 221], and His saying: ﴿Neither are these [believing women] lawful for them [disbelieving men] nor are those [disbelieving men] lawful for them [believing women]﴾ [Al-Mumtahanah: 10]. There is also a risk of her falling into disbelief. The guardian of a woman married to a disbeliever should advise her, and if she does not respond, the matter should be referred to the judiciary in a Muslim country. Otherwise, he should continue advising and reminding her. He is not permitted to kill her, as changing a reprehensible act by killing is not for individuals, but for those in authority, lest a greater tribulation occur. If she apostatizes from Islam or was previously married (thayyib) and her brother kills her, he is mistaken, but no retribution (qisas) is applied to him because her blood is not inviolable (ghayr ma'sum ad-dam), and the brother should be punished (ta'zeer). If she did not apostatize and was a virgin, then retribution (qisas) is applied to her killer. If qisas is not applied, then he must offer sincere repentance and supplicate for her extensively.