There’s a big difference between having some type of mental or personality disorder, and knowing right from wrong.
Someone who doesn’t understand right from wrong would have been more likely to not attempt the abduction under the cover of darkness, stealthily sneaking into the tent IMO shows a knowledge that he should not have been there, the presence of mind to be quiet. Many people can hold down a job, drive etc and still have some sort of mental illness or cognitive issue. I’m not saying he’s insane, let’s get that clear.
Im trying to think of an alternative motivation for this, probably because I desperately don’t want there to have been a sexual motive. I’m not saying there wasn’t one, as I said ‘IF the motive wasn’t sexual then it could possibly be.. x y z’
Being brought up by his grandmother could suggest some childhood trauma issue, when you think of the reasons someone might be raised by a grandparent instead of a parent the reasons seem to only involve something that would have affected the child at some point, death, neglect, abandonment, abuse etc.. however that is never an excuse for committing crimes against children, plenty of people experience childhood trauma and manage to go through life without a single brush with the law.
As you mentioned it is very telling that he has no links with his family or any friends in the neighbourhood. Not everyone who isolates themselves has a mental illness, but it is a suggestion that he might have some type of avoidant personality, attachment disorder, or even paranoia that makes him not trust anyone. There are lots of possibilities here, as we don’t have much information to really build up an idea of who he is, but we can try.
JMO MOO