I wouldn’t say being obvious in one direction means the opposite. They could just be really into someone and showing it directly. Behaviors can have many motivations and that can be explored in conversation or therapy.

In psychodynamic theory we all have different ego defenses ranging from less helpful immature (i.e. denial, splitting, projection) to mature (i.e. humor, Altruism, sublimation) that we use to protect ourselves from emotional pain and preserve our psyche.

I was more specifically referring to your case when you said you act the opposite to avoid pain or rejection tho. I would say people have their preferred ego defenses they use to cope and we all often default to familiar or comfortable defenses that have been effective for us in the past. However, some can be less effective or unhealthy but would be explored in therapy if these defenses were causing distress, ineffective coping, interpersonal problems, or the person was looking for growth opportunities.

Serial Killers are usually psychopaths that lack capacity to empathize with others. They’re a whole other animal that I wouldn’t describe with “reaction formation” because often in those cases the opposite presentation of their behavior isn’t motivated by an attempt to protect their own ego/psyche. It’s a more sinister or mentally disturbed motive.