Was Al-Khidr's killing of the boy due to the righteousness of his parents? Does Allah command a prophet to kill a human being? And why did Allah not cause him to die instead of sending a prophet to kill him?

The Law of Al-Khadir (peace be upon him) differs from the Law of Musa and from our Law. Musa disapproved of Al-Khadir's actions because they did not align with his Law. The killing of the boy was by Allah's command, out of fear that the boy would burden his parents with tyranny and disbelief, for if he had lived, he would have led them astray with his disbelief, so Allah replaced him with someone better. This verse illustrates the easing of calamities, such as the loss of children. Some scholars found Al-Khadir's killing of the boy problematic, so it was said that he was an adult deserving of death due to his disbelief or because he was a highway robber. It was also said that he was a non-pubescent child, and Al-Khadir knew, by Allah's revelation, that if he were to reach adulthood, he would be a disbeliever who would mislead his parents. This issue is one of the matters of divine decree, which should not be delved into.