Why did God create the infidel boy, knowing that he would harm his parents, and then command that he be killed, instead of not creating him and bestowing upon his parents a believing boy?

Allah is wise in His management, and He possesses ultimate wisdom in what He decrees for His servants. It is obligatory to submit and acknowledge that all of Allah's actions are good and wise. Scholars have mentioned several reasons for the wisdom behind killing the boy, including that he was an adult and an unbeliever, or a highway robber, or that accountability in the law of Al-Khadir was not conditioned on reaching puberty, or that he was killed to prevent his harm and evil, like snakes and wild beasts. Among the wisdoms resulting from his killing are stopping his evil from reaching his parents and society, and testing the parents, which would bring them great reward if they were patient, and replacing him with someone better. As for the objection to creating him and then killing him, Allah created His servants for His worship, and He punishes whoever disobeys Him. This increases the reward of the prophets, callers to Islam, and mujahideen in enforcing hudud punishments and suppressing corruption.