Lilli ☕, that depends on your plot. The antagonist is the person or thing that causes conflict for your main character. If your serial killer is trying to get front row seats to the opera but the box office insists those seats are reserved for season ticket holders, then your antagonist is either the box office clerk, or his manager, or the box office agency's policies, or possibly the season ticket holder who happens to be in your main character's desired seat, if the show's about to start. The fact that she's a serial killer is not relevant unless that's relevant to your plot.

So... what's your premise? What happens to your serial killer main character in this book?

Cheers,
Michelle