@Solipsist : He is a Paragon figure. He inspires others, even unintentionally.
It's not a 'moral of the story' story, but a showcase of how much of an inspiring figure he is.
He told the kid that the job come with having to kill people.
But seeing him kill a monster that would've otherwise injured/killed the people there, the kid is inspired to take up the path even knowing that.
Because that's the 'ideal knight' he's aspired to be.
One that protect the people by killing monsters.
Which if you'd recall, is exactly the kind of job knights of Tersia family do:
https://mangadex.org/chapter/331399/6