What does the title have to do with the movie?

Actually nothing.

Wikipedia:

Modelled after the infamous Richard Speck and Ted Bundy murders, 10 to Midnight uses a screenplay originally named Bloody Sunday. According to producer Pancho Kohner, Cannon Films chairman Menahem Golan and Kohner named the film 10 to Midnight despite having no connection to the plot.

Golan and Kohner had intended to film an adaptation of the R. Lance Hill novel The Evil That Men Do, which fell through before an upcoming visit to the Cannes Film Festival. Golan and Kohner agreed to market a different film with Charles Bronson as its star, using 10 to Midnight as its working title.

While the title really has nothing to do with the movie proper, whenever this question comes up I point people to the poster artwork for the film. Notice the killer and his hand positions. One hand is straight up (the 12 o'clock/midnight position), while the other, holding a knife, is in the 10 o'clock position. Thus, "10 to Midnight".

according to the commentary on the 2015 Twilight Time Blu-ray the Producers sold a movie to Golan/Globus with JUST A TITLE and the idea for Bronson to star.

Later they found a script called "Bloody Sunday" and renamed it "10 to Midnight".

That's it....the title has nothing to do with the plot of the film.

If you listen to the commentary this is all told within the first four minutes.

I will not fear. Fear is the mind killer.

Good question.

Only thing I can think of its a plot hole