If they did not come out of the womb with an egocentric personality like Oppositional Defiant Disorder or develop the early warning signs of Childhood Conduct disorder, chances are they have either witnessed traumatizing events, been subjected to trauma themselves, or there truly is something they are hiding from you.
[Note: Children who start acting out by late Elementary or early Middle School years have oftentimes experienced being bullied at home or sexually abused. Resist the urge to minimize negative behaviors or to back down about the need to set healthy boundaries with children of this age; bad tempers are not normal no matter what Enabler friends try to tell you. The very same person trying their hardest to convince you that a child’s behavior is age-appropriate is the one most likely to have situationally and covertly abused the youth.]
If they are truly toxic by nature and are biologically incapable of feeling empathy, their egocentrism lends itself to the development of NPD (Narcissistic Personality Disorder) in later life. By the time kids turn 18 they can, technically speaking, be diagnosed — but truly, it is between the age of 18 and 28 that the personality fully forms, calcifying their nature into something people who are not afraid to make pop culture references in the field of psychology might refer to as a “head case”.