Reading through the chat logs has given me a new insight into Nick. I get the impression that there was some thing(s) that happened between him and his mother as a child which lead to a lack of bonding between the two. Maybe she couldn't figure out how to deal with a toddler so just let him do whatever and withdrew from the relationship, maybe she had post natal depression and neglected Nick.
It would explain his fixation on butts and pooping - at the potty training stage in their lives children get a lot of attention for going in the toilet. What if Nick in his disturbed mind craves that positive attention he got from his mother for pooping in the toilet and turned that into an obsession with shit? He knew he got attention for the poo so now he smears it on walls and himself. It would have generated more attention from his mother. Over time this has evolved from wanting attention from his mother into a full blown scat fetish. Pure speculation of the Freudian kind, of course.
Nick's constant "but I didn't do it!" excuse is probably also a learned childhood thing. I have seen kids who were so convincing at lying to their parents and the parents so badly wanting to give them the benefit of the doubt that they take it and do nothing about it. The kids thinks all he has to do is say he didn't do it and the situation will remedy itself. Never being taught consequences or what is right or wrong (and why it is right or wrong) is going to fuck up a child. Which leads me to:
The sexual behaviour he exhibits with Maddi in the chat log is more disturbing. The worry for me is that he is attempting to convince another person to fellate him because it's enjoyable - not considering
the other person may not be interested or find it enjoyable
. I have read a few case files on sexual offenders who have said similar things: but the other person gets enjoyment from it too, I didn't nothing wrong, the other person will like it if they try it. I feel that the ones that think this way are the worst offenders because they don't understand/empathise with the person on the other end and project that because it is enjoyable for themselves it will be enjoyable for everyone else. Maddi hits then nail on the head when she accuses Nick of grooming - the real danger here is someone who doesn't think what they are doing is wrong or harmful and will influence someone who doesn't know any better to participate, i.e. a child. (thank goodness for Maddi being intelligent and seeing through it - you did good, kid).
Couple all this together and there is a perfect storm for Nick to turn into a mental deviate who will attempt to convince others to participate in things for his gratification. As I said, all Freudian speculation, but if Nick has a history of childhood head trauma (falling from swings, bikes, etc) then this could lead to his brain not being able to develop the parts that lead to rational thinking and empathy.
(sorry for the Tl

R, I've been lurking for so long and it has kinda just all burst out).