Does Al-Khidr's killing of the boy mean that Al-Khidr changed his destiny, or was the boy "created a disbeliever" as stated in the Hadith: "The boy whom Al-Khidr killed was created a disbeliever, and had he lived, he would have oppressed his parents with tyranny and disbelief"?

Predestination (Qadar (definition)) is Allah's foreknowledge, His inscription of it, and His creation according to His will and desire. No one can change it. Al-Khidr did not act on his own in killing the boy; rather, it was by Allah's command. This indicates that Allah's decree for the believer is always good, even if its apparent aspect is disliked, as Allah Almighty says: "And perhaps you dislike a thing and it is good for you." As for changing predestination, it is confined to what is in the Angels' scrolls, not in Allah's knowledge or the Preserved Tablet (Al-Lawh Al-Mahfuz). Supplication (Du'a) and maintaining kinship ties (Silat Ar-Rahim) can alter what is written in the Angels' scrolls, such as increasing lifespan or preventing illness, but Allah has known all of this eternally.