A live-action TV version for the controversial webtoon The Elementary School Student That I Love has been canceled because it's about an elementary school teacher sexualizing a 5th grader

To me, Léon feels more Don't Stand So Close To Me than My Sharona, which is to say that in the former song, the male character recognizes his attraction is inappropriate and in the latter the guy is proclaiming, " Never gonna stop, give it up, such a dirty mind. I always get it up, for the touch of the younger kind. "

More explicitly, Léon has a moral objection to killing woman and kids. He isn't looking to lure in children for exploitation, he just wants to get paid to murder adult men. His morals are not our morals. While he absolutely falls for Mathilda, I see no evidence that Léon has a pattern of sexualizing kids.

I can see your point of view that by teaching Mathilda how to assassinate people Léon was in fact grooming her, but he was not the one coaxing his too-young neighbor to hide in his house and learn his trade to avenge her family. She is making these demands of him and he is initially reluctant. That is not to say his attraction to her is appropriate. He is a villainous 'hero', and like so my of the archetype, must pay for his years of immorality with his life.

TL;DR: I think you stretch the definition of 'Grooming' when the adult is trying to keep distance and the child makes the demands.