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  

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  

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  

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.'" (Using translation from Tirmidhī 3150)   

نَبِيُّ اللَّهِ ﷺ «إِنَّ الْغُلَامَ الَّذِي قَتَلَهُ الْخَضِرُ طُبِعَ يَوْمَ طُبِعَ كَافِرًا»