OK, think it's about time I tried typing out my 'pet' theory. I will preface this by saying there are other far simpler explanations for the crime than what I'm about to suggest below and I do lean towards those explanations way more than this one.
This is just a theory I formulated for reasons I can't fully put into words, just call it a vibe I get from everything we've seen
All of the below is my own opinion and pure speculation!
The why:
Simply, they wanted to. No financial motivation, no revenge, no obsession. They wanted to plan out and commit this heinous crime because the planning and execution of the crime excited them.
Why NG?
This is where my theory gets a bit 'out there' in terms of speculation. My thinking for why they might have chosen NG is that they have done this before. Maybe not the kidnapping, but the planning and execution of a similar enough crime. But importantly, with someone a lot lower profile than NG. Possibly someone from their local area, maybe that crime was simply put down to a burglary gone wrong.
But the next time, they needed something more, they needed more excitement. So they chose a higher profile target, maybe they saw that TV segment and that's how they got to NG or they simply looked up notable people from Tucson on Wikipedia, which includes SG and they went from there.
But the point I'm making is, they have no tangible connection to NG.
Thoughts on the suspect
Again, this is major speculation on my part as there's not a whole lot to point to this, but in this theory, the suspect is likely someone relatively young (perhaps in their late 20s or early 30s) who thinks they are smarter than everyone else and spends too much time on True Crime sites online (hell they've probably been on here asking oddly specific question). They think they can commit the perfect crime and outsmart everyone.
They spent a huge amount of time planning and trying to account for every possibility
They got a gun and holster and wore it in a way that's intended to be obvious and threatening to an old lady even if it looks ridiculous to most who have experience with firearms
Maybe they somehow sourced (or made) a device to jam the camera's WiFi signals, they planned their route in and out of the area carefully to avoid major roads/intersections, they watched NG to learn her routines, they cased the house thoroughly, maybe they even went out of state to rent a car to use in the crime, instead of turning their phone off they simply left it behind at home next to their bed, a reasonable location for it at that time of night.
They got a cheap 'disposable' outfit, they wore multiple layers of masks and gloves to reduce the possibility of DNA transfer. They used a bite-light so their hands were free without thinking of the risk of potential increased DNA transfer because their bite-light idea was so "brilliant"
Then on the night of the crime, things didn't exactly go to plan. The battery on their jammer died and the got surprised by an online camera they thought should have been offline and grabbed some plants to try block it because once things don't go to plan, this guy panics.
Maybe the first couple of ransom notes were really this guy, it wasn't really about the money, NG was sadly already long dead but hey, he was smart enough to fool the police, why not try make a bit of money out of it. But once he realised those videos made it to the cloud, his confidence got knocked and he got cold feet.
In Summary
The crime was the point, the perp wanted an escalating thrill so choose a high profile (but soft) target. He looked amateur because once things went off script, he couldn't handle it.
Again, this is all my own opinion and pure speculation. This is so far from being the simplest explanation for the crime, it's just a feeling I got from everything we currently know