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.
Sahih Muslim Book 46, Hadith 45
← Previous Hadith It is Allah alone Who knows what they would be doing according to their creation.

Sahih Muslim Book 46, Hadith 44 Next Hadith → There is happiness for this child who is a bird from amongst the birds of Paradise. Thereupon Allah's Messenger (ﷺ) said: Don't you know that Allah created the Paradise and He created the Hell and He created the dwellers for this (Paradise) and the denizens for this (Hell)?

Sahih Muslim Book 46, Hadith 46