Their mental capacity to understand western ways and values will find no place in their minds and hearts saturated with hate, violence, abuse and indoctrination from infancy on both by the culture itself, which accepts such, and within the home.
In most cases whatever personality they were designed to have is ‘destroyed’ and replaced with the “cultish” thinking we so often observe and why it's almost impossible to reason with the average Muslim. Most of the Islamic fighters are childlike in every way......easily led, emotionally unstable for what they experience...and why drugs are “used” to ease the otherwise resemblance that might spark of a conscious..which is past just being seared.
I'll also add that if they attend Mosque, and go deeper into the Koran.....that will assuredly produce an inner trigger which could erupt in a given situation.