Depends on their motive. If a serial killer kills just for fun, well that means the person is either insane or escaped a mental hospital. That person themselves went coconuts. Thus, in my book not only it becomes romancitizing serial killers but also romanticise psychopaths.
Let’s not talk about it toxic or not, it just doesn’t make sense for a sane person to fall in love with an insane person without being brainwashed.
If you’re writing a realistic story… Don’t.
Different story for people who kill with different motives. Now, technically assassins don’t fall into the “serial killer” category but illegally taking lives is still considered murder/manslaughter.
You can romanticise people, NOT their occupation (or what they do for a living because well… You get it…). You can fall in love with someone who’s secretly a killer. But don’t make them love the killer.
The problem with those, coughs toxic mafia stories is that the MC loves the LI for they are the mafia leader, not because the LI is someone the MC cherishes, despite being a mafia leader. You see, the LI should be a person worth loving, not the LI being the mafia leader, not the LI being a serial killer. Heck, not even the LI being a millionaire, or a teacher, a student, a bartender, argh you name it!!!
Because if you make the MC love the LI’s status instead of their person as a whole…
You make them no different from desperate h*es.
Thanks for attending my Ted talk. Nova out.