what I can remember:
most of it played in a diner, where the murderer (female, young, black hair) falls in love with a guy.
They were randomly meeting there and just talking (if I remember right)
She kills (or poisons) unfaithful (I guess married) men in hotel rooms / apartments.
Something about the black widow killer came to my mind, but nothing found through google
I would place it probably around the 90s
Hope this little description helps
Thx in advance!
Can't remember that film's name? Post the plot, and see if other users can help.