﴿ فَأَرَدْنَآ أَن يُبْدِلَهُمَا رَبُّهُمَا خَيْرًۭا مِّنْهُ زَكَوٰةًۭ وَأَقْرَبَ رُحْمًۭا ﴾

“and so we desired that their Sustainer grant them in his stead [a child] of greater purity than him, and closer [to them] in loving tenderness.”

(And We intended that their Lord should change him for them for one) another child (better in purity) righteous (and nearer to mercy) who is more keen to keep ties of kinship. Allah gave them instead of him a girl who later married a prophet and from whom she gave birth to a prophet at whose hand a great number of people were guided. The son killed by Khidr, whose name was Jaysur, was, on the other hand, a disbeliever, a thief and a murderer. This is why Khidr killed him.