terrible movie

That does NOT make it a terrible movie,

It only makes it: “a plot that can’t be understood.”....
Think about that.

For me, it's depicting a nightmarish modern world where cultured, civilized people have little chance of survival, while brute force, sadism, and destruction are the reigning values. Mike Hammer is the best "hero" that such a world can offer, and there's very little to differentiate him from his enemies insofar as they regard the world & other human beings. The plot is a dizzying pursuit of a poisonous Grail that destroys rather than restoring & healing. And like a nightmare, it makes little logical, rational sense ... but emotionally, viscerally, it makes its killing point.