I'm going with the theory that Egon was genuinely suffering from some mental illness issues, there's a fine line between genius and madness and Egon was such a genius but also such an eccentric that I don't think it's too surprising that as he got older he may have run into some issues and wasn't thinking clearly.
Yes, it's a sad fate for the character, but the actor had a sad fate too, but at least Egon did help save the day and was able to reconnect with everyone.
I don't think that's fair to Egon's character though. I mean if that had been the case it would've made more sense, but the film plays it as if his teammates didn't believe him about the coming apocalypse. Like after all the shit they've seen, why would they doubt Egon on this? The smartest guy on their team and who built everything they used? I think that's Afterlife's weakest point which could've been handled way better. I mean they could've had it so the other teammates went their own separate ways after their business dried up like how they did in Extreme Ghostbusters, and just have it so Egon left for farm country to deal with this new upcoming threat while the other ex-ghostbusters wished him the best, but didn't go with him because they were too focused on maintaining their new careers and families. Its a better alternative than having Egon be mentally ill and just running off with the equipment because the other guys didn't believe him, especially Ray of all people.
Still, even while I don't like how the film repeats too many beats from the first film, especially the finale and having the same villain, or how its just a pseudo Stranger Things x Ghosbusters crossover, among other things, the theme of Egon going solo was explored once before in the Extreme Ghostbusters cartoon, which is actually pretty great and I recently found out that it still gets referenced by Sony, even in some Cisbusters 2016 tie-ins; so if that was their intent for why they had Egon go solo, then its a nice little nod, but not one that was executed as well as the cartoon, which to be frank while good, I still find the idea of all the ghostbusters (specifically Ray) splitting up and leaving Egon alone to be a bit unbelievable. I mean Peter and Winston leaving would make perfect sense, especially when the money stopped coming in, but Ray was just as obsessed with the science of the supernatural as Egon; so if anyone would've stuck by Egon aside from Janine, it would've been Ray, which is where I think both Afterlife and Extreme fall short. Even in Ghostbusters 2 when they were forced out of business, Ray and Egon were the only ones still obsessed with the occult research (although In Ray's case it was mostly through his book store), while Winston was just trying to keep his job with Ray and Peter had since moved on.
Now while the following isn't a defense of Afterlife's split up plotline or any ideas of Egon being mentally ill, I guess I could see Egon's obsession with ghost hunting being a borderline schizo obsession, since if we go by past Ghostbusters media, they had it so the reason an empirical guy like Egon is the way he is and delved into the supernatural in the first place was because of a traumatic encounter with a Class VII specter when he was a child which eventually led to his goal of understanding and combating the supernatural himself, something that's still referenced in new Ghostbusters media and tie-ins, including the updated versions of Tobin's Spirit Guide which are mostly written from a live-action perspective. I doubt the Afterlife writers put that much forethought into the reasoning though.
But making Egon straight up mentally ill and a sad wreck near the end of his life for no reason over such a silly reason as his fellow ghostbusters not believing him, which they have no reason not to after all they've seen, is just ridiculous.
Also while I thought the Walmart appearance felt like shameless advertising and that the mini-stay pufts felt a bit like a forced attempt at marketing new toys, the latter is at least something that has some precedence, since small chaotic offshoots of Gozer/Stay Puft are almost a recurring thing outside the movies, so I guess there was a reason for them (although the game gave them a nice spooky look). That's the most I'll play Devil's Advocate.
I still stand by my earlier review of the film being around 6/10 or 7/10, but its not as rancid as rlm and their fanboys are making it out to be, but it is definitely flawed and too unoriginal in some parts. Would've enjoyed it more if they had used another separate entity as the main villain or at least one related to Gozer, as well as given Egon better reasoning for his actions and the decline of the Ghostbusters, and that they had used a lookalike for Ramis instead of a cgi zombie. As for the rlm review everyone keeps talking about, I ain't going to watch that shit, but if they're being extra cynical about it, then that's the sort of cynicism they should've used when TFA came out since it did most of what this film did but worse, with more cynicism and was even more disrespectful to the past elements and characters, to the point where some of the people who worked on the DT confirmed that they didn't even watch the films, at least not more than once. Anyway its best if you decide for yourself if the movie is good or bad in the end and base your opinions on either what you saw (legally or through yohoho) or what you read or hear from people you trust or can speak with rather than what e-celebs say.
TL;DR:
As far as my opinion goes, the movie was not as offensive to the senses as other "30 years later" sequels to 80s films and it has some amusing moments and the kids are alright, but too many plot points were too repetitive for their own good, the cast was giving off Stranger Things 4.0. vibes more than they needed to, and Egon's character arc and return as a ghost could've been handled way better. 6.8/10 - Its okay and fun for the kids.