By necessity, a serial killer would need to hide their activities to keep from being caught, jailed, and possibly put to death. But a character may also harbor this secret to protect the important relationships in their life and maintain a facade of normalcy. While there are some commonalities between serial killers, they're also driven by different motivations, fears, personality factors, and mental health considerations. So if this secret pertains to your protagonist, build that character carefully to make sure all the pieces align.

Dark or Disturbing

BEING A PSYCHOPATH, BEING A SOCIOPATH, FANTASIZING ABOUT KILLING SOMEONE, HOLDING SOMEONE CAPTIVE, KNOWING WHERE A BODY IS BURIED, MURDERING SOMEONE, PLANNING A MASS MURDER OR TERRORIST ATTACK

BEING PITIED, CERTAIN KINDS OF PEOPLE, HUMILIATION, LOSING ONE'S MIND, NOT BEING IN CONTROL, PERSECUTION, REJECTION, RELATIONAL COMMITMENT

Not receiving help that could improve their mental health
Desiring relationships with others but being unable to relate to them emotionally
Always feeling isolated because no one truly knows them
Prioritizing flying under the radar, which keeps them from pursuing certain professional or personal goals
Being paranoid about getting caught, always looking over their shoulder
Being unable to trust others because they themselves are untrustworthy
Spiraling into self-loathing and self-hatred (if they're able to feel these emotions)

Adopting normalizing habits to help them fit in with others so they don't stand out
Feigning empathy and other emotions that aren't natural for them
Marrying and developing friendships to maintain a semblance of normalcy
Compartmentalizing: keeping their desires and criminal activities distinctly separate from their day-to-day life
Creating cover stories to hide where they've been and what they've been doing
Lying frequently and believably
Using charm to disarm people
Only targeting people they perceive as weak and easy to control or subdue
Having a fake identity and false papers
Avoiding patterns in their criminal behaviors to make it harder to get caught
Being morally flexible so they can do whatever is necessary to keep their secret hidden
Expressing their dark desires through mental fantasies rather than talking about them
Having a secret or private room, drawer, box, space, etc. to hold tools, documents, and trophies

Being caught in multiple lies
Cruelty to animals
The character always being “out of town” when killings happen
Keeping suspicious supplies in the car (duct tape, rope, tarps, etc.)
Having a morbid fascination with a certain method of killing (poisons, knives, hanging, etc.)
Keeping a private area that no one is allowed access to (a second apartment, a storage unit, closet, safe, etc.)
A stash of trophies being discovered
The killer having knowledge of the crimes that they shouldn't have

Increased scrutiny (being a witness in legal case, achieving success that garners media interest, a suspicious spouse hiring a private investigator, etc.)
An injury or illness that makes it harder for them to successfully subdue a victim
Being questioned by police
Heightened security measures that make hunting people more difficult
A serious problem at home that requires the character to be more present

Almost getting caught
A family member finding something the character wanted to keep hidden (a burner phone, weapon, trophy, etc.)
A spouse pushing the character to attend couples therapy to deal with issues in their marriage
Developing empathy, making it hard to continue killing
A growing subconscious desire to get caught

Someone finding out the truth and blackmailing the character (to stop killing, pay them off, etc.)
Going to jail and losing autonomy
Being sentenced to death
Losing all the relationships they had built, and being completely alone
Being viewed as despicable by others
Not being able to act on their compulsions
Having to go on the run and live in hiding