other movies that together launched the 'slasher' genre which completely destroyed the horror genre
I don't think this is true. This is like saying that one sub-genre ruined the whole thing, like saying vampire fiction ruined horror fiction. There are plenty of non-slasher horror films out there during the golden age of horror (dates to be determined, but probably began in the early- to mid-seventies and ended when
Scream came out).
Although a few good movies have come out in the past decade or so, I think that industry standards have ruined horror.
1. Remakes, sequels, and sure bets run the market currently, despite the surge in the popularity of horror. Everyone's afraid of making something new. No one wants to lose money.
2. The stricter guidelines for admittance to movies (I know that under 17 shouldn't be in an R movie, but they made it in). Horror movies have always been geared to a teen and young adult audience. There was an understanding that younger teens 13-16 would find their way into theatres, but now, with less making it in, they dumbed down horror to PG-13, which in many cases doesn't work in the film's favor.
I don't think slasher ruined horror, it was just a popular sub-genre. I think money ruined it. Money and a lack, or resistance to, ingenuity.