Innovative filmmaking
I think Malle is by far the most underrated director/writer in European history. That last scene you realize is as much catharsis as horror. I wouldn't call this his best movie, but if the film succeeds it's pulling the movie out at the last second from a dopey coming of age drama to a weird black comedy. Because he was a versatile director and played it so straight you were never sure where it was going. (Today if it was made, you'd know going in it was an adult comedy because it would have an indie soundtrack and a poster with Michael Sera, etc., wearing his mothers' dress, completely giving away the film by emphasizing a throwaway scene.)
And Malle makes characters that can be repulsive but still endearing. Bunuel and Godard, who I'm pretty sure both had equally privileged upbringings, failed to really ever make a case against the bourgeoisie. They hated rich people and tried desperately to make you hate them too by making them utterly absurd, stiff clowns. Malle, on the other hand, doesn't seem to be preoccupied with judging everyone. With the possible exception of Black Moon, I haven't been let down yet by him. Never understood why his work was overshadowed by Truffaut or the others of his time. He was head and shoulders above them.