What I love about the film is that up to the final scene, it's kept ambiguous as to whether Michael Myers is just a homicidal lunatic, or if as Loomis suspects there's something genuinely unnatural and "evil" about him.
For the final scene when Loomis looks over the balcony and finds Michael's body missing, Donald Pleasance had asked Carpenter if he should react with shock or if he should react with the expression of "I always knew this would happen." Carpenter said "do whatever feels natural." So Pleasance went with the grim expression that indicated "he knew this would happen" and that his suspicions about Michael's true nature were correct. Capping it off with a quick succession of location shots where Michael had killed people overlayed with Michael's heavy breathing and Carpenter's score made it perfect.
Of course that was all tossed out in Halloween II when we see the previously calm but grim Loomis running about screaming how he couldn't believe Michael was alive, completely undercutting this awesome ending.