I don't like gory movies as a rule, and this one has multiple axe murders. Not many, but still, there are multiple axe murders. This one is a grim film.
Internet Archive has it online:
1000 Misspent Hours concludes their review by saying this:
Go in expecting the cult classic that its reputation would seem to suggest, and you’re likely to be disappointed. What you’ll be getting instead is a promising beginning to a career, but not much more than that.
In truth Dementia 13 is simply an enjoyable B-movie, one of the numerous psycho-thrillers that were being made around the time by Hammer and William Castle. Coppola is clearly copying the success of other psycho-thrillers ... Considered in itself, Dementia 13 is only a fairly minor entry in this genre.
The New York Times review from the time of the film's release says,
Under the stolid direction of Francis Coppola, who also wrote the script, the picture stresses gore rather than atmosphere, and all but buries a fairly workable plot.