Nick's behavior wasn't that out of character for him. Go back at episodes like "Alter Boys," "Caged", etc. and you'll see that he does have a tendency to settle on the first suspect he meets.
You are correct, but what bothers me about this is that Nick years ago, when he was younger and less experienced, used to think like that, and Grissom always corrected him, mentored him, tutored him. And Nick learned from it and grew into a great CSI...Grissom's "best student" as he said himself.
To put him back to this level of...narrowmindedness? It's not good writing, in my opinion. I'm surprised George went along with it considering the premise for this show was his idea and he watched the actual "Hoarders" show and knows a lot about how it works with people who have this problem.
You got to remember Smokey, it's George who watches the show and knows how it works - not Nick. You could tell between the beginning whe he says (and I paraphrase) "how can people live like this" and the end when he's cleaning out his desk that he had a very hard time coming to terms with the fact that someone could not only do this to themselves but to their children (and the son sounded like it had gone on since they were kids).
Besides Nick didn't come to his conclusion and just stop at I'm right and you're wrong.
Just because the mother was mentally disabled does not make her incapable of killing someone. It may make her incapable of understanding killing someone but not incapable of doing it.
Did Sara ever talk to the case worker? Ultimately I put most of the blame on the case worker. She was, obviously, not following the case close enough.
Susan