I don't know if it was my mood, but I think this is a better film than folks make it out to be. Certainly, it depends on your criteria. But compared to other slasher films, it stands out. It's true that the killing only happens right at the end, but I like how the murders serve the plot as opposed to vice versa. And, certainly, anyone paying attention could have figured out who the killer was- but I tend to watch films like this in the moment, so I'm usually as suprised as the characters when the big reveal happens. The most astounding moment in the film is when Jamie Lee Curtis's character (Kim) looks into the killer's eyes and knows who it is. At that moment, I knew who the killer was as well. Not because I'd put all the clues together and figured it out, but because the look on Curtis's face so perfectly portrayed the shock and betrayal she felt, there's no way I could have thought it was anyone else. That moment is realistic and chilling because of it.
The other part that affected me was the slight subplot involving Lou, the poor chap who gets [spoiler] beheaded instead of Nick. I thought the scene earlier in the film (Leslie Nielsen as a serious actor- wow, not bad) where the principal basically dismisses the guy, admitting that he plays favorites with his son, was about as scary as the murders at the end. There's a statement in there about the educational institution and kids' powerlesness in it. Granted, Lou is asking for it because he's a bully, but as we all know (again, me reading into characters what's probably not there in the film but would be in real life), bullies are hurting more than most on the inside. Lou gets snubbed by his principal, used by Wendy to get back at her ex-boyfriend and then [spoiler] beheaded. He was a throwaway character from the start, and while this is common in slashers from the eighties (you can always tell who's going to die first), it seems especially pronounced in Prom Night because of how Lou is set up to be the star attraction. He's the prom king now, in a mean-spirited but relatively harmless prank...but only for a moment before the ax falls. Of all the characters who died, he is the only one who is actually innocent of the crime for which he is being punished.
Kelly's fate was also particularly tragic. She's been rejected by her boyfriend for not putting out and is sitting in the dark in tears when she is attacked and killed. Kelly, like Nick, was very hesitant to keep their childhood secret, timid creature that she was. But Nick gets to stay alive at the end. Who could fault Kelly, at such a tender age, for succumbing to peer pressure? Yet her death is one of the darkest in terms of the multitude of complex emotions she would have had to endure before her consciousness dimmed. It's also notable because often in these films characters are killed while having or having had sex. I can't think of too many where the character being killed has just essentially refused sex.
Curtis's acting is compelling at the end as well. She portrays powerfully her loss as she cradles the killer's head in her hands, crying out from the depths of her heart.