All kids go through brain development (Behtereva N wrote brilliant books about child brain development). However, more than 99% of kids do not commit serious crimes, certainly do not murder innocent people. Kids tend to know right and wrong.
With regards of rarety of phycopathic behaviours - tbh I can't see the contradiction with what I said / provided data before.
ASPD develops only on youth younger than 15 y.o., generally before they turn 11. Once developed ASPD is not curable.
The cause of ASPD is not definitive. Most authors believe that it is caused by a combination of upbringing, serotonin level and genetics.
ASPD is a lifelong disorder with poor prognosis (Salekin, R. T., 2016; Bruce, M et all, 2015) due to low remorse or regret and consequent inability to see the impact of antisocial behaviour. Generally, people with ADPD tend to simulate remorse to get out. 78% people with ADPD commit crimes again (Fisher et all, 2019).
There is no ASPD cure unfortunatelly (my.clevelandclinic.org/health/diseases/9657-antisocial-personality-disorder#management-and-treatment). Medications and CBT can control symptoms to some extend, but would a person with ADPD take medications?
Who knows, may be those two boys/killers will be an examplery exception.
I do feel sorry for Shawn and his family. He was only 19.