Al-Khidr the Allah-Taught and His Victim

In hadith he's something of a big noise.

"Ubai bin Ka`b narrated to us that Allah's Messenger (ﷺ) said, 'Moses got up to deliver a sermon before Bani Israel and he was asked, 'Who is the most learned person among the people?' Moses replied, 'I (am the most learned).' Allah then admonished Moses for he did not ascribe all knowledge to Allah only (Then) came the Divine Inspiration:-- 'Yes, one of Our slaves at the junction of the two seas is more learned than you.'...When they reached the rock, they found a man covered with a garment. Moses greeted him. The man said astonishingly, 'Is there any such greeting in your land?' Moses said, 'I am Moses.' The man said, 'Moses of Bani Israel?' Moses said, 'Yes,' and added, 'may I follow you so that you teach me something of the Knowledge which you have been taught?' (18.66). Al-Khadir said to him, 'O Moses! You have something of Allah's knowledge which Allah has taught you and which I do not know; and I have something of Allah's knowledge which Allah has taught me and which you do not know.'...Then they both proceeded and found a boy playing with other boys. Al-Khadir took hold of him by the head and cut it off. Moses said to him, 'Have you killed an innocent soul who has killed nobody? Surely you have done an illegal thing! ' (18.74)"

https://sunnah.com/urn/44060

S 19:74, 80 Then they set out until when they met a boy; then he killed him. He said: Have you killed a pure soul for other than a soul? You have done an abominable thing.

And as for the boy his parents were believers and we feared that he would weary them with oppression and kufr.

Who were the "we" who feared the boy's wearying his parents with oppression and kufr?

The Jalalayn on this murder by the Allah-taught unnamed Al-Khidr:

So they set off after leaving the ship making their way on foot until when they met a boy who had not yet reached puberty playing with other boys among whom his face was the fairest — and he al-Khidr slew him by slitting his throat with a knife while he lay down or by tearing his head off with his hand or by smashing his head against a wall all of which are different opinions the coordinating fā’ of fa-qatalahu ‘and he slew him’ is used here because it indicates that the slaying took place after the encounter; the response to idhā ‘when’ is the following statement qāla … — he Moses said to him ‘Have you slain an innocent soul that is a pure one that had not reached the age of legal responsibility a variant reading for zākiya has zakiyya one slain not in retaliation for another soul? in other words one that has not slain any soul. Verily you have committed an dreadful thing’ read nukran or nukuran that is to say an abomination. https://www.altafsir.com/Tafasir.asp?tMadhNo=0&tTafsirNo=74&tSoraNo=18&tAyahNo=74&tDisplay=yes&UserProfile=0&LanguageId=2

In another hadith we read:

Narrated Ibn 'Abbas:

that Ubayy bin Ka'b narrated that the Prophet (ﷺ) said: "The boy that Al-Khidr killed was destined to be a disbeliever the day he was created.'"

https://sunnah.com/urn/641690