Surah 18. Al-Kahf, Ayah 81

The son was practically an outlaw,-a danger to the public and a particular source of grief to his righteous parents. Even so, his summary capital punishment would have been unjustified if Khidhr had been acting on his own. But Khidhr was not acting on his own: see the latter part of the next verse. The plural "we" also implies that he was not acting on his own. He was acting on higher authority and removing a public scourge, who was also a source of extreme sorrow and humiliation to his parents. His parents are promised a better-behaved son who would love them and be a credit to them.