The project manager sounds like a dick, but the author also sounds like a push-over. The author had a chance to out the manager as a huge asshole, but based on the post, it seems like he just left silently.
Personally, I've deeply appreciated a lot of the reviews I've seen of employers on Glassdoor. Just recently, during my last job hunt, I was looking into a couple San Diego companies with promising positions...until I looked them up on Glassdoor. When I saw not one, but many reviews from different reviewers saying such things as "Working for this place will suck your soul dry then crush it" I immediately lost all interest in working for the company. Granted, the Glassdoor reviews aren't perfect, which is why I look for general agreement on the company from different reviewers and/or position titles (as a proxy for different departments of the company).
But sometimes it is funner to burn all bridges.
The only thing the OP should have done differently, IMO, is made clear to his colleagues that he handed his notice and that he wasn't being sacked.
to quote.. """, it allowed the transactions of users to be skewed, allowing malicious users to cause other users to essentially lose money, or worse yet, put them in a potential dispute, and ultimately, cause our product to lose customers."""
report to the regional financial services authority. Create anonymous story about how company x is causing massive user vulnerabilities etc.Send this information to the companies clients.You could cause massive damage and come out looking like the good guy if you spun it right.