The boy whom al-Khidr had killed was created an infidel. Had he lived, he would have moved his parents to rebellion and unbelief.
قَالَ رَسُولُ اللَّهِ ﷺ الْغُلاَمُ الَّذِي قَتَلَهُ الْخَضِرُ طُبِعَ كَافِرًا وَلَوْ عَاشَ لأَرْهَقَ أَبَوَيْهِ طُغْيَانًا وَكُفْرًا
The young man whom Khadir killed was a non-believer by his very nature and had he survived he would have involved his parents in defiance and unbelief.
قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ الْغُلاَمَ الَّذِي قَتَلَهُ الْخَضِرُ طُبِعَ كَافِرًا وَلَوْ عَاشَ لأَرْهَقَ أَبَوَيْهِ طُغْيَانًا وَكُفْرًا
that Ubayy bin Ka'b narrated that the Prophet ﷺ said: "The boy that Al-Khidr killed was destined to be a disbeliever the day he was created.'"
عَنِ النَّبِيِّ ﷺ قَالَ الْغُلاَمُ الَّذِي قَتَلَهُ الْخَضِرُ طُبِعَ يَوْمَ طُبِعَ كَافِرًا
[Machine] About the Prophet Muhammad ﷺ , the boy who was killed by Al-Khidr was sealed as a disbeliever. Abu Al-Rabi'i added in his narration that if he had lived longer, he would have caused great oppression and disbelief to his parents.
عَنِ النَّبِيِّ ﷺ الْغُلَامُ الَّذِي قَتَلَهُ الْخَضِرُ طُبِعَ كَافِرًا زَادَ أَبُو الرَّبِيعِ فِي حَدِيثِهِ وَلَوْ أَدْرَكَ لَأَرْهَقَ أَبَوَيْهِ طُغْيَانًا وَكُفْرًا
"الْغُلام الَّذِي قَتَلَهُ الخَضِرُ طُبْعَ يَوْمَ طُبعَ كَافِرًا وَلَوْ عَاشَ لأَرْهَق أَبَوَيهِ طُغْيَانًا وكُفْرًا" .