Watch it again sometime. It would be rated R. Or whatever the equivalent is these days.
I only needed to see it once. I was underage at the time and had to sneak in the back door of the theater. One of my 18 year old friends bought a ticket and the rest of us went around to the back exit and waited for him to have a chance to open the exit door.
That movie left a mark. It was typical of the time when all the movies of the late 60's left you depressed as hell when you left the theater.
These days I get depressed when I see a movie because the only adjective that screenwriters know how to use starts with an f and ends with a k.