This came to me last night. I remember the main gimmick being that a few people actually became their Halloween costumes. I only really remember two. A guy with a samurai sword attacks the people with him and commits seppuku when defeated by the cops, or whoever came to the rescue. (It may have been another person with a sword-themed Halloween costume, but not a possessed person.) Another guy who was dressed as a jester actually laughs himself to death as his horrified girlfriend watches, like with blood spitting out of his mouth and everything.
It's possibly Neon Maniacs. There are both Samurai and Jester "Neon Maniacs", and the ending takes place at a Halloween dance.
Neon Maniacs part 8 (http://www.youtube.com/watch?v=MAldA5EZcik#ws)
Hi,
Unhelpful part: it sounds like a 'Treehouse of Horror XVI' episode from 'The Simpsons'.
Hopefully helpful part: This is based on an episode of 'The Twilight Zone' called 'The Masks'. Here's the Wikipedia link: https://en.wikipedia.org/wiki/The_Masks
Also, there was an episode of "Buffy the Vampire Slayer" with a similar theme. It doesn't fit the description so probably not it, but may be worth checking just in case (or, you want to watch other stuff with similar theme).
Season 2, Episode 6, "Halloween." http://buffy.wikia.com/wiki/Halloween (http://buffy.wikia.com/wiki/Halloween)
Was it a kids movie. Seems like I remember something similar that my kids watch every year at Halloween. Maybe a Disney movie?
Try HALLOW'S END ('03, DTV) http://www.imdb.com/title/tt0338088/ (http://www.imdb.com/title/tt0338088/)
A similar thing happens in NIGHT OF THE DEMONS 2 (97) http://www.imdb.com/title/tt0116073/ (http://www.imdb.com/title/tt0116073/)
Welp, move to Solved.
It seems I was thinking of both Hallow's End and Neon Maniacs. Could be that my friends and I marathoned some Halloween-themed movies and they got mushed together in my imagination.