Could be the first slasher film...

The modern slasher film supposedly started with Psycho, and it seems true enough, but I would say this film is a better example of the slasher genre, for these reasons:

1. Opening sequence establishes the mood with a knife murder

2. Flashback sequences help the setup

3. Young people place themselves in an eerie setting, no help nearby

4. Killings are of young people (mostly) involved in sexual activity

5. Various killing methods, but typically using sharp instruments

6. The further investigation leads to more killing

7. High number of deaths, in total, eleven

8. Deranged human killer with deformity

9. Killer is protecting his home from trespassing youths

10 Anti-climactic surprise ending just when they think they've got the killer

11. Homage to Psycho, in this case, dead and decaying Mom in a chair suddenly appears.

12. Youths constantly go places where nobody in there right mind would go

13. Hot girls and nudity

HAs pretty much all the typical standard fare that we see in later slasher films.

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The Eyes of the City are Mine! Mother Pressman / Anguish (1987)