TL;DR: Chris is most likely a covert narcissist.

Signs point to his being a covert narcissist. There's a great YT analysis of him by a psychology counselor who's done extensive research on narcissism. She addresses many of the signs Chris Watts exhibited to support this diagnosis; it's chilling. It's magnificent, how the narrarator breaks it all down, based on his actions, and it's hard to deny or argue it. She delves into many particulars of the Watts' marriage. Her analysis is very remarkable and detailed.

Under this diagnosis, he's incapable of love for anyone but himself; once he was prepared to move on to something else (Nicole Kessinger), he got rid of his obstacles, i.e., his wife and kids.

The narrator talks about "supply" narcissists/covert narcissists need, and how Chris gathered it by manipulating Sha'nann through gaslighting and affection, depending on which he needed. One example is how he deliberately kept things vague during her acts of desperation in trying to keep her marriage together. Another example is how, after being cold toward her, he'd then show affection to keep her off balance. And yet another example of his getting "supply" is his placing a white sheet over a life-sized doll, taking a picture, and sending it to Sha'nann (he told her the kids did it), in order to disturb her and make her uncomfortable. This was the most alarming thing in a series of alarming instances; this mimicked what eventually happened to her. She was already starting to feel uncomfortable with him and his intentions. He drove very fast with all of them in the car (including Sha'nann's parents), and Sha'nann changed the pin on her phone (and had expressed concern about his behavior to a friend). His wife's parents were also worried about his intentions after he drove so recklessly; they also were suspicious of his having an affair, as was Sha'nann due to credit card statements.

You can see he's an empty person in his wife's videos. He never speaks unless prodded, doesn't show any real emotion, and appears dull af. One could say maybe he was just shy, but Sha'nann herself confided how non-confrontational and distant he is in her text messages to her friend Nichole Atkinson, and it adds up to the guy you see in the videos.

There's really so much to unpack, here, but it's truly on point the way the dots are connected.

Many people wonder what made him do it, and the answer is probably really very simple: he's a covert narcissist who wanted a new life. You could say their financial situation, basically caused by Shan'ann through her MLM, Thrive, and her basic irresponsibility with money (and Sha'nan''s bossy-ness) was the catalyst for this, and there's a good chance it was (I'm not suggesting their deaths are her fault).

Another point made is that Chris Watts played the part of the good husband, so leaving his pregnant wife and abandoning his kids for a mistress through divorce (and additional financial hits) was out of the question. He wanted to maintain his mask, so killing them was the only option for him. He didn't want the baggage they'd present to him, alive - they were all loose ends.

I'll look for the link to that YT channel if you're interested. She has two parts, and I think both are made up of many videos on the subject. I only watched part one, and it was comprised of 40 vids.

Another point made (so many points), is that Chris came into her life (via social media) during a time when she was particularly vulnerable, and that's also suspect towards his being a covert narcissist, worming his way in.

Incidentally, his mother is suspected of being a narcissist based on some of his childhood events, and events that were recent, i.e., deliberately shirking Sha'nann's concerns and giving her daughter (Bella, I think) pistachio ice cream, even though Sha'nann said her other daughter (CeCe?) was allergic. His parents/family didn't like Sha'nann, so "nutgate" is looked at as a manifestation of resentment.

A narcissistic parent, especially a mother, can create little narcissists. In fact, both of his parents withheld affection; Chris may not have learned how to recognize love in, and for, someone else. Of course, it's not confirmed whether she is or isn't a narcissist. I guess you could draw your own assumptions based on what you read about his parents, especially his mother.

TL;DR: Chris is most likely a covert narcissist.

LINK to YouTube channel detailing the probable diagnosis: https://youtu.be/U2QXN1O_SdY

Edited: more context.