How do you know he was 2 when adopted?
One of the relatives who remained unnamed stated that in a news article and that the boys had the same birth mother. At the same time, there was a photo on Lynda Cruz’s obit page with her and a newborn. That baby was identified as Nikolas, perhaps wrongly.
The relatives had very little interaction with the family. Four people attended her funeral - the 2 sons and neighbor Rocxanne & her ex boyfriend.
It would be revealing to confirm from where Nik actually originated and this could confirm the armchair diagnosis that appears to strongly fit his behavior and actions.