Like about a few hundred other films from the early to mid 70's. They were dark times and the films reflected that.
That is one of the things I love about movies from that time, including this one. They could get away with endings that were dark and/or ambiguous. Nowadays, if it isn't a happy (or at least optimistic) ending, it's a cheap shock ending. And you don't get much ambiguity unless it's blatantly setting up a sequel. Back then, you could do it for artistic reasons.
Saw this movie on TV a few months ago. It was the first time I'd seen it in probably close to 30 years. Not as much tension as I remember, but that's true of most thrillers I saw as a kid.
It did remind me of how good Martin Sheen was at playing smarmy creeps.