A woman stands up one man for another because he is nicer, so she thought. She soon discovers he’s a serial killer and her life depends on the first man to send that psycho to hell.

The "protagonist" has no consequences stacked up against her. She also has no goal. Why is she doing this? Readers can already tell she'll be a flat character.

WHO THE STORY IS ABOUT, WANTS SOMETHING VERY BADLY BUT MUST DO WHAT TO GET IT.

Agree with Richiev.

You gotta decide who owns the story.? The woman who foolishly stands up the nice guy in favor of the wrong guy only to realize.. ? Or the nice guy who maybe was too nice, who has to rise to the occasion and become a mensch instead of a milquetoast (character arc) in order to rescue the damsel in distress who stood him up?

Whoever owns the story -- which is to say, the one with the stronger character arc --? frame and focus the logline around that character as the protagonist.

fwiw

Plenty of conflict here. I like the concept.
Just make sure to follow the basic logline formula so you can pack all the information in one sentence.